DECCAN TRAP IRON-CLAY. 201 



of the western coast laterite), are in many places of the most unques- 

 tionable sedimentary origin. Although so much evidence exists in 

 favor of the sedimentary origin of the fringe of ferruginous deposits which 

 surround the southern part of the Indian peninsula, it is impossible to 

 be quite certain as to the nature of the rock Dr. Buchanan named 

 " laterite/"* till the Malayalam country shall have been examined by some 

 competent geologist and that question set at rest. Till that point 

 is definitively settled, the name laterite can only be provisionally 

 employed, and should be applied only to rocks of sedimentary origin. 

 Such being the case, a very large proportion of the argillo-ferruginous 

 rocks in the South Mahratta Country and South- Western Deccan, now 

 regarded as not of sedimentary origin, though lateritoid in character, can 

 no longer be treated of under the old name of laterite, but the term 

 will be retained for certain petrologically similar rocks in the Southern 

 Konkan, which, from their geographical situation, may reasonably be 

 supposed to be of sedimentary origin, and possibly also to belong to a 

 later geological period. For the others a new name must be found, 

 and it will be simplest to revive the designation given by the late Dr. 

 Voysey to the corresponding rocks in the centre of the Deccan at and 

 around Bidar, viz., " the iron-clay,^^ — a name at once brief and 

 lithologically correct in its description of the rocks in question. Dr. 

 Voysey was the first to point out the great probability of the " iron-elay^^ 

 having originated by decomposition of a truly trappean rock — a view 

 which the facts now to be given appear to prove conclusively true. 



Certain other argillo-ferruginous rocks, apparently of lacustrine 

 origin, which occur in the lower valleys of the Malprabha and Ghat- 

 prabha rivers, will also be classed as laterites. 



Among the iron-clays it becomes very important, though by no 



means always easy, to distinguish two classes, — Istli/, those which must 



be regarded as members of the Deccan trap series much altered from 



their pristine condition by subaerial agencies ; and %ndly, those which 



2 b ( .201 ) 



