928 JOURNAL, BOMBAY NATURAL HISTORY SOCIETY, Vol. XVI TI. 



No. XXXII. -SOME NOTES ON THE LOW-LEVEL LATERITE 

 OF PORTUGUESE INDIA. 



This rock is so well known as to require no detailed description. 



In its normal form, it has been described as a porous argillaceous rock, much 

 impregnated with iron peroxide irregularly distributed throughout the mass. 



Its lateral distribution is fully treated in the Manual of Geology of India. 



The origin of the rock appears to be still unknown and it remains a debat- 

 able point, whether laterite is detrital or merely the result of the decomposition 

 " in situ " of the underlying rock. 



The main argument in favor of the latter view is that the two rocks are 

 seen to pass into each other. 



All high level laterite, or those deposits of the rock found at a high elevation 

 on the plateau of the Deccan and elsewhere, is supposed to have originated 

 from the change of its underlying rock, generally a trap, while the low level 

 type of the Konkan and East Coasts is generally granted to be of detrital origin. 



While the writer was in Portuguese India, on a short geological tour, he had of 

 necessity to traverse a considerable area of what is known as the low level 

 laterite and his observations on its relation to the rocks immediately under- 

 lying, in many places, from Chapora southwards along the coast to Cabo de 

 Rama and eastwards towards Sanvordem, may be of interest. 



Near Chapora, along the sea coast, the laterite forming the main escarpment, 

 was seen to lie unconformably on the tilted up and eroded edges of highly 

 indurated shales and sandstones, associated with sills and dykes of basalt. 



It extended inland, covering some fairly high hills in the neighbourhood of 

 Assangao. 



There appeared to be no passage from the laterite downward into the 

 underlying rocks. 



East south-east from Marmagao, in the neighbourhood of Chandor, some 

 hills, composed mainly of quartzites, rise out of the laterite covered plain, 

 which is here slightly higher than the coast escarpment. 



No trace of the laterite was found on the hills, which are well above the 

 level of the laterite deposit, nor was there any sign of passage from one 

 rock into the other observed at their point of contact. 



Between Assolnam and Cabo de Rama on the Coast and in several places 

 in the interior, where the laterite is found overlying the traps, the passage of 

 the lower rock upward into the laterite is well seen. No sharply defined line 

 at point of junction between the laterite and trap was noticed, the former 

 rock having the appearance of having been formed by the change of the 

 underlying trap. 



The occurrence of several pieces of fossil wood, impregnated with the iron 

 peroxide, in blocks of laterite cut in the neighbourhood, point, however, to the 

 detrital origin of the rock in spite of the appearance of its having been formed 

 • in situ. " 



