184".] including Notices of Sumatra, the Malay Peninsula, fyc. 521 



geologists concluded that plutonic action was necessarily deeply subter- 

 raneous. But here, I think, we find a subaerial or subaqueous plu- 

 tonic activity ; and where the plutonic level has not reached that of the 

 pre-existing rocks, a new kind of metamorphism appropriate to the new 

 conditions under which the plutonic exhalations have operated. 



The interest which the discussions respecting laterite have given to 

 that rock, tends to invest it with undue importance geologically. The 

 ferruginous 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 alteration. 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 iron-masked 

 rocks of the Indo- Australian regions. This term will include the prin- 

 cipal or plutonically ferruginated rocks, which, without being either 

 completely reduced or metamorphosed, have been either wholly dis- 

 guised or partially altered by ferruginous emissions, which have saturat- 

 ed 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 disconuity, the sides of 

 which have an imperfect cohesion, or having a common border of infe- 

 rior density and increased porosity caused either by interruptions in 

 the original deposition of the matter of the rock or by unequal stretch- 

 ing or incipient cleavage. The term may be also extended, perhaps, to 

 those sedimentary beds in which the iron saturation, although coeval 

 with the deposit of the other constituents of the rock, has served to 

 obscure or conceal their true nature as well as the derivation of the 

 beds themselves. These beds appear to have been sometimes formed 

 by superficial layers of gravel, &c. being permeated by iron solutions. 

 With these must not be confounded the broad bands lying over and 

 beside the heads of iron-masked dykes, and which, having been in a 

 loose gravelly or fragmentary state at the time when the plutonic emis- 

 sions passed through them, became cemented into hard, and occasional- 

 ly scoreous, ferruginated conglomerates, &c. and are therefore proper 

 plutonically iron-masked rocks.] 



3 y 2 



