Volcanic Islands

outh

one fo

eudo-fragme

ifica

nt-cl

t va

emen's

oic for

ion with vol

h leaves of e

on of t

Zeal

eorge'

al ferrug

eous deposits, wit

rifted particles of

r ex

f Good

he granite an

one fo

pe of Good Hope. In order to confine the Third Part of these Geological Observations to South America,

OUTH

high at the edge of the escarpment, and rises in a distance of twenty-five miles to between three and four thousand feet above the level of the sea. At this distance the road descends to a country rather less elevated, and composed in chief part of primary rocks. There is much granite, in one part passing into a red porphyry with octagonal crystals of quartz, and intersected in some places b

coarser materials than the lower: this appears to indicate that, as the sea became shallower, the force of the waves or currents increased. On the lower platform, however, between the Blue Mountains and the coast, I observed that the upper beds of the sandstone frequently passed into argillaceous shale,-the effect, probably, of this lower space having been protected from strong currents during its elevation. The sandstone of the Blue Mountains evidently having been of mechanical origin, and not having suffered any metamorphic action, I was surprised at observing that, in some specimens, nearly all the grains of quartz were so perfectly crystallised with brilliant facets that they evidently had not in their PRESENT form been aggregated in any previously existing rock. (I have lately seen, in a paper by Smith (the father of English geo

ng been deposited, and before being worn away by the currents. Each patch of the shale shows, also, how slowly many of the successive layers of sandstone were deposited. These pseudo-fragments of shale will perhaps explain, in some cases, the origin of apparently extraneous fragments in crystalline metamorphic rocks. I mention this, because I found near Rio de Janeiro a well-defined angular fragment, seven yards long by two yards in breadth, of gneiss containing garnets and mica in layers, enclosed in the ordin

ION OF THE

render it probable, that we here see an original escarpment, not formed by the sea having eaten back into the strata, but by the strata having originally extended only thus far. Those who have been in the habit of examining accurate charts of sea-coasts, where sediment is accumulating, will be aware, that the surfaces of the banks thus formed, generally slope from the coast very gently towards a certain line in the offing, beyond which the depth in most cases suddenly becomes great. I may instance the great banks of sediment within the West Indian Archipelago (I have described these very curious banks in the Appendix to my volume on the structure of Coral-Reefs. I have ascertained the in

NT-CL

ples, the tops of which have since been cut off, either by nearly horizontal layers, or by another set of great ripples, the folds of which do not exactly coincide with those below them. It is well-known to surveyors that mud and sand are disturbed during storms at considerable depths, at least from three hundred to four hundred and fifty feet (See Martin White on "Soundings in the British Channel" pages 4 and 166.), so that the nature of the bottom even becomes temporarily changed; the bottom,

THE SANDSTO

f pale sandstone. The bottoms of these valleys are moderately level, and the fall of the rivers flowing in them, according to Sir T. Mitchell, is gentle. The main valleys often send into the platform great baylike arms, which expand at their upper ends; and on the other hand, the platform often sends promontories into the valley, and even leaves in them great, almost insulated, masses. So continuous are the bounding lines of cliff, that to descend into some of these valleys, it is necessary to go round twenty miles; and into others, the surveyors have only lately penetrated, and the colonists have not yet been able to drive in their cattle. But the most remarkable point of structure in these valleys, is, that although several miles wide in their upper parts, they generally contract towards their mouths to such a degree as to become impassable. The Surveyor-General, Sir T. Mitchell, in vain endeavoured, first on foot and then by crawling between the great fallen fragments of sandstone, to ascend through the gorge by which the river Grose j

on the present coast of New South Wales, which are generally connected with the sea by a narrow mouth, from one mile to a quarter of a mile in width, passing through the sandstone coast-cliffs, present a likeness, though on a miniature scale, to the great valleys of the interior. But then immediately occurs the startling difficulty, why has the sea worn out these great, though circumscribed, depressions on a wide platform, and left mere gorges, through which the whole vast amount of triturated matter must have been carried away? The only light I can throw on this enigma, is by showing that banks appear to be forming in some seas of the most irregular forms, and that the sides of such banks are so steep (as before stated) that a comparatively small amount of subsequent erosion would form them into cliffs: that the waves have power to form high and precipitous cliffs, even in landlocked harbours, I have observed in many parts of South America. In the Red Sea, banks with an extremely irregular outline and composed of sediment, are penetrated by the most singularly shaped creeks with narrow mouths: this is likewise the case, though on a larger scale, with the Baha

EMEN'S

d have been described by him: they consist of two species of Producta, and of six of Spirifera; two of these, namely, P. rugata and S. rotundata, resemble, as far as their imperfect condition allows of comparison, British mountain-limestone shells. Mr. Lonsdale has had the kindness to examine the corals; they consist of six

could judge, is at least a thousand feet in thickness: the upper few hundred feet usually consist of a siliceous sandstone, containing pebbles and no organic remains; the inferior strata, of which a pale flinty slate is perhaps the most abundant, are the most variable; and these chiefly abound with the remains. Between two beds of hard crystalline limestone, near Newtown

capped by streams of basaltic lava with olivine; and close by there was a mass of brecciated scoriae, containing pebbles of lava, which probably marks the place of an ancient submarine crater. Two of these streams of basalt were separated from each other by a layer of argillaceous wacke, which could be traced passing into partially altered scoriae. The wacke contained numerous rounded grains of a soft, grass-green mineral, with a waxy lustre, and translucent on its edges: under the blowpipe it instantly blackened, and the points fused into a strongly magnetic, black enamel. In these characters, it resembles those masses of decomposed olivine, described at

WITH EXTIN

y composed), nor do they resemble that class of exceptions to the common form of the leaves of the Eucalyptus, which occur in this island. The travertin containing this remnant of a lost vegetation, is of a pale yellow colour, hard, and in parts even crystalline; but not compact, and is everywhere penetrated by minute, tortuous, cylindrical pores. It contains a very few pebbles of quartz, and occasionally layers of chalcedonic nodules, like those of chert in our Greensand. From the pureness of this calcareous rock, it has been searched for in other places, but has never been found. F

ON OF T

to a small elevation of the land. On the shore of Ralph Bay (opening into Storm Bay) I observed a continuous beach about fifteen feet above high-water mark, clothed with vegetation, and by digging into it, pebbles encrusted with Serpulae were found: along the banks, also, of the river Derwent, I found a bed of broken sea-shells above the surface of the river, and at a point where the water is now much too fresh for sea-shells to live; but in both these cases, it is just possible, that before certain spits of sand and banks of mud in Storm Bay were accumulated, the tides might have risen to the height where we now find the shells. ( It would appear

va, and scoriform rocks, forming distinct craters;-2nd, A castellated hill of horizontal strata of flesh-coloured limestone, showing when fractured distinct crystalline facets: the rain has acted on this rock in a remarkable manner, corroding its surface into a miniature model of an Alpine country: I observed here layers of chert and clay ironstone; and in the bed of a stream, pebbles of clay-slate;-3rd, The shores of the Bay of Islands are formed of a feldspathic rock, of a bluish-grey colour, often much decomposed, with an angular fracture, and crossed by numerous ferruginous seams, but without any distinct stratification or cleavage. Some varieties are highly crystalline, and would at once be pronounced to be trap; others strikingly resembled clay-slate, slightly altered by heat: I was unable to form any decided opinion on this formation.) Whatever may have been the origin of these shells, I cannot do

EORGE'

form of the bare conical hills, appearing to be composed of great folding layers, strikingly resembles, on a small scale, those composed of gneiss-granite at Rio de Janeiro, and those described by Humboldt at Venezuela. These plutonic rocks are, in many places, intersected by trappean-dikes; in one pla

AL FERRUG

ywhere coated either by a fine-grained, rubbly, ochraceous mass, like that at King George's Sound, or by a coarser sandstone with fragments of quartz, and rendered hard and heavy by an abundance of the hydrate of iron, which presents, when freshly broken, a metallic lustre. Both these varieties have a very irregular texture, including spaces either rounded or angular, full of loose sand: from this cause the surface is alwa

L CALCAREO

es here have had a similar origin. The impalpable powder has probably been derived from the decay of the rounded particles; this certainly is possible, for on the coast of Peru, I have traced LARGE UNBROKEN shells gradually falling into a substance as fine as powdered chalk. Both of the above-mentioned varieties of calcareous sandstone frequently alternate with, and blend into, thin layers of a hard substalagmitic rock, which, even when the stone on each side contains particles of quartz, is entirely free from them (I adopt this term from Lieutenant Nelson's excellent paper on the Bermuda Islands "Geolog. Trans." volume 5 page 106, for the hard, compact, cream- or brown- coloured stone, without any crystalline structure, which so often accompanies superficial calcareous accumulations. I have observed such superficial beds, coated with substalagmitic rock, at the Cape of Good Hope, in several parts of Chile, and over wide spaces in La Plata and Patagonia. Some of these beds have been formed from decayed shells, but the origin of the greater number is sufficiently obscure. The causes which determine water to dissolve lime, and then soon to redeposit it, are not, I think, known. The surface of the substalagmitic layers appears always to be corroded by the rain-water. As all the above-mentioned countries have a long dry season, compared with the rainy one, I shoul

tres are often filled with black carbonaceous matter not yet removed. It is not surprising, that the woody matter should have been almost entirely removed from the casts on Bald Head; for it is certain, that many centuries must have elapsed since the thickets were buried; at present, owing to the form and height of the narrow promontory, no sand is drifted up, and the whole surface, as I have remarked, is wearing away. We must, therefore, look back to a period when the land stood lower, of which the French naturalists (See M. Peron "Voyage" tome 1 page 204.) found evidence in upraised shells of recent species, for the drifting on Bald Head of the calcareous and quartzose sand, and the consequent embedment of the vegetable remains. There was only one appearance which at first made me doubt concerning the origin of the cast,-namely, that the finer roots from different stems sometimes became united together into upright plates or veins; but when the manner is borne in mind in which fine roots often fill up

taining animal matter, as shown by their evolving ammonia. Had Dr. Macaulay seen the enormous masses of rolled particles of shells and corals on the beach of Ascension, and especially on coral-reefs; and had he reflected on the effects of long-continued, gentle winds, in drifting up the finer particles, he would hardly have advanced the argument of quantity, which is seldom trustworthy i

uch in appearance from these at Bald Head, and the other places above specified.) It appears also from M. Peron, with whose observations and opinions on the origin of the calcareous matter and branching casts mine entirely accord, that the deposit is generally much more continuous than near King George's Sound. At Swan River, Archdeacon Scott states that in one part it extends ten miles inland. ("Proceedings of the Geolog. Soc." volume 1 page 320.) Captain Wickham, moreover, informs me that during his late survey of the western coast, the bottom of the sea, wherever the vessel anchored, was ascertained, by crowbars being let down, to consist of white calcareous matter. Hence it s

F GOOD

d by clay-slate: the latter is generally hard, and glossy from containing minute scales of mica; it alternates with, and passes into, beds of slightly crystalline, feldspathic, slaty rock. This clay-slate is remarkable from being in some places (as on the Lion's Rump) decomposed, even to the depth of twenty feet, into a pale-coloured, sandstone-like rock, which has been mistaken, I believe, by some observers, for a separate formation. I was guided by Dr. Andrew Smith to a fine junction at Green Point between the granite and clay-slate: the latter at the distance of a quarter of a mile from the spot, where the granite appears on the beach (though, probably, the granite is much nearer underground), becomes slightly more compact and crystalline. At a less di

rm N.W. and S.E. cleavage. This fact has been observed in other similar cases, and has been advanced by some eminent geologists (See M. Keilhau "Theory on Granite" translated in the "Edinburgh New Philosophical Journal" volume 24 page 402.), as a great difficulty on the ordinary theory, of granite having been injected whilst liquified; but if we reflect on the probable state of the lower surface of a laminated mass, like clay-slate, after having been violently arched by a body of molten granite, we may conclude that it would be full of fissures

ack brilliant mica, and of few and thin laminae of quartz. From the abundance of the mica in this gneiss, compared with the small quantity and excessively minute scales, in which it exists in the glossy clay-slate, we must conclude, that it has been here formed by the metamorphic action-a circumstance

ONE FO

and perfect crystals, are very numerous; and it is evident in nearly all the strata, that silica has been deposited from solution in remarkable quantity. Many of the varieties of quartzite appeared quite like metamorphic rocks; but from the upper strata being as siliceous as the lower, and from the undisturbed junctions with the granite, which in many places can be examined, I can hardly believe that these sandstone-strata have been exposed to heat. (The Rev. W.B. Clarke, however, states, to my surprise ("Geolog. Proceedings" v

sandstones of the great platforms of Eastern Australia, which also rest on granite, differ in containing more earthy and less siliceous matter. No fossil remains have been discovered in these three vast deposits. Finally, I may add that I did not see any boulders of far-transp

VOLCANI

reous casts at the

don i

lands, incr

plosions at

driven from

arle

he Galapagos

al cells,

, calcareou

cks of. -absence of dikes, freedom from v

, extin

an, new volc

te,

tra

or

Brazil,

the mountains

d H

s' C

he, St.

pecific g

ns at Mauritius. -at S

in lava. -on dikes indicating elevation. -on i

calcareou

-on laminated trachyte. -on obsidia

o

, vol

Vincent,

in Australia and

le is

calcareo-animal substan

from Van Diemen's land. -on sphae

nking in obsidian. -on laminated lava. -on obsidian streams. -on oli

ed in streaks in scoriae. -freestone at Ascension. -incrustations at Asce

f Good

d, expulsion

t., on glassy c

careous, o

donic

basalt and in

am is

roph

., on the Cape

tion and junction with gr

o

clay-slate i

cross, in

dation at

nar b

ount of volcanic phen

on, eart

nd igneous rocks compare

e, recent,

, curiou

l, from Van D

-great central one at St. Helena. -i

levation. -of tuff at Terceira. -of tuff at the Galapagos archipelago. -thei

tion favour

M., on sp

in-formed island. -on

. -on the composition of obsidian

esia in erupted lime. -on sp

of coast at

Peak, St

Dr., on the C

glossy layer; uniform thickness of. -great parallel ones at St. Helena. -not observed at Ascension.

t Ascension. -

n of volcan

d trachyte. -on lamina

crystals si

the surface of certain lava-streams.

ed at St. Helena. -o

Ascension. -at the

-Van Diemen's Land, Cape of Good Hope, New Zealand,

in the great crater at Hawaii.

n, fiss

land-shells

the expulsion of

f. -in radiating cryst

a. -rock, alternating with obsid

ndo N

us superf

areous matte

s of er

, on calcar

Hill, St

Bellevue on

ty of

r, on the struct

Ascension. -at the

water

eurteventura), c

pelago. -parape

he expulsion of

, their

of texture,

clay-slate. -with a g

ite, form o

ope, C

rrow, at S

th clay-slate, at th

ejected f

specific,

anic strata at St. Helena. -on

the expulsion of

of, on calc

, on ashes

rof., on c

n decompose

Dr., on

eo-animal substance. -on

, Dr.,

n obsidian formations. -on parape

on amyg

decompose

cation of the cir

, distribution of

n, on St. P

, calcareous,

o,

s is

, orig

u de, on craters

Ferna

M., on

er R

eorge'

feldspar,

bases of

n of volc

extinct, at

calcareou

a gorge. -feldspathic. -w

ecific gr

s in the state of their surfaces. -extreme thinness of. -heaved up into hillocks at

aration f

on craters a

uc

hate of, a

fossil-corals from

St. H

tion. -on embedded turtles' egg

on calcareous c

gdaloid. -on chlorophaeite

ms. -on glossy coatings to dikes. -on obsi

calcareou

account of volcanic ph

eka

crater of e

-origin in metamorphic s

abrador feldspar. -on quart

on bombs. -on the

t the Galapag

ough i

., on the Ber

Cale

stone, cross

outh

Zeal

ssil), resembl

n and origin of. -crystals of feldspar sink in. -its irruption from l

-at Van Diemen's land. -in the l

f recent calcareous

hei

, extin

s, laminated

n the separation

St., r

rls

eri

calcareous roc

-laminated. -with mor

one. -d

s, ex

ation of constituent

o Pr

ity of great dislocati

s hill, S

the Galapagos arch

ome, tra

sland,

an. -crystallised in sandstone. -fusibility of. -roc

hi

e altered

aneiro,

on strata

r, on curved lin

pared with

ea. -in volcanic strata

e Cape of Good Hope. -platf

, radi

bsidian. -on separation of trachyte and basa

St. Helena. -on dikes. -

inct shells o

ofessor, on

n concretio

on uprais

che

Diemen's land. -land, extinct, at St. Helena.

r deposited

., on r

l Pos

-large proportion of, in obs

eous

junction of gran

on decompo

nt calcareous rocks and

ss and in silicifie

from Van Diemen's land. -from St.

-crater of e

ation of. -effects of ca

aul's

tions of sphaerulit

top, L

re

of sandstone in

s of o

, on marine rem

space at

hi

fied, within

cei

deposit o

ra. -decomposition of, by steam. -its lamination. -its separation from b

lutonic series. -at

at Van Die

now rare, a

their breached stat

the separation o

ett on marine re

St. Helena. -in New So

emen's

rachyte. -

Bory St.,

ess of formation in the Atlanti

s passage

argill

sin-formed island. -o

rtin, on

of, on the f