$8 
SKETCH OF THE PHYSICAL GEOGRAPHY 
and other ironmasked* rocks, which constitute the most characterise 
tic features, and the chief interest, of the sedimentary rocks of the 
southern part of the Peninsula.*!* 
In other cases the platonic action has indurated the superincum* 
bent strata, converting sandstones into compact siliceous rocks, and 
clays and conglomerates into chert and other hard crystalline forms. 
The appearance of the ironmasked and ironribbed strata in ridges 
and branched ranges, dipping sometimes towards the S. W. and some¬ 
times towards the N. E., generally at high angles, often vertical or 
nearly so, occasionally horizontal, or moderately inclined, and with 
all these variations occurring in limited localities, impresses the spec- 
* “ The interest which the discussions respecting laterite have given to 
that rock, tends to invest it with undue importance geologic ally. The fer¬ 
ruginous emissions have affected all rocks indiscriminately, and their action 
on sandstones, grits and conglomerates is as well marked as that on clays, 
marls and shales, although the latter only produces proper laterite. Even 
in the clays, laterite denotes one only of many degrees and forms of altera¬ 
tion. To express the origin of these rocks, and its unity, to record the cause 
of the difficulties which they have presented, and to distinguish them from 
true metamorphic rocks, I would propose, avoiding any new technical names, 
to term them simply the ironmasked rocks of the Indo-Australian regions. 
This term will include the principal or plutomcally ferruginated rocks.'which, 
withont being either completely reduced or metamorphosed, have been ei¬ 
ther wholly disguised or partially altered by ferruginous emissions, which 
have saturated them in the mass,—or only affected them in fissures and seams, 
—or been interfused between portions of the rocks not actually separated by 
fissures, but intersected by planes of mere discontinuity, the sides of which 
have an imperfect cohesion, or having a common border of inferior densi¬ 
ty and increased porosity, caused either by interruptions in the original de¬ 
position of the matter of the rock, or by unequal stretching, or incipient 
cleavage. The term may be also extended, peihaps, to those sedimentary 
beds in which the iron saturation, although coeval with the deposit of the 
other constituents or the rock, has served to obscure or conceal their true 
nature as well as the derivation of the beds themselves. These beds ap¬ 
pear to have been sometimes formed by superficial layers of gravel, Arc. be¬ 
ing permeated by iron solutions. With these must not be confounded the 
broad-bands lying over and beside the heads of ironmasked dykes, and 
which, having been in a loose gravelly or fragmentary state at the time when 
the plutonic emissions passed through them, became cemented into hard, 
and occasionally scoreous, ferruginated conglomerates Arc. and are there¬ 
for proper plutonically ironmasked rocks.” Introduction to paper On the 
Local and Relative Geology of Singapore fyc. (cited ante p. 92.) [March, 
1847.) . . 
-J- The igneous rocks of the Peninsula, viewed by themselves in particular 
localities, prove that a difference of pressure is not essential to ihe contem¬ 
poraneous production of plutonic and volcanic types, and the general pluto¬ 
nic phenomena of the zone further seem to us to shew that granite bubbles 
may swell to the surface of the sedimentary rocks,—which they assimilate 
and absorb while they raise. The greater part of the sedimentary crust of 
the Peninsula has not, according to our view*, been removed by denudation. 
