Sec. 16.] detailed descriptions. 213 



This conglomerate is nsually more or less nodular, and occasionally 

 the bed appears chiefly made up of nodular concretionary pebbles, 

 which, when weathered, strongly resemble casts of large univalve 

 shells. The matrix of the pebbles which form the bulk of the rock 

 is a coarse sandstone, containing small rounded fragments of agate 

 and quartz, rarely exceeding an inch, and generally below 5 inch 

 in diameter. 



The second principal variety is the same coarse sandstone with agate 

 pebbles, the latter, however, being neither numerous nor conspicuous, 

 without any of the rounded blocks of sandstone. 



Both varieties appear to be calcareous ; they are firm compact rocks 

 and excellent building stones. The wall surrounding part of the island 

 is built of them, and they have been largely quarried for grindstones, 

 and also for exportation as building stones. The principal quay at Surat 

 is also built of them. 



In the conglomerates, besides bones, immense masses of fossil wood 

 occur, very hard, heavy, and black, sometimes, but not always, having 

 been bored by Teredo. No sandstone blocks bored by PholadidcB, like 

 those in the Taptee bed, could be found, although they were especially 

 searched for. 



In places fine sandstones occur, red or brown in colour, and generally 

 in beds from 6 inches to a foot thick. Such beds, if consolidated and 

 then broken up, might furnish the sub-angular blocks of the conglo- 

 merate. 



The beds are in general horizontal. The sections given by 

 Ethersey convey a fair idea of the strata in the low cliff at the south end 

 of the island, but these beds are so variable that no two sections would be 

 likely to give precisely the same result. It is doubtful if the strata be 

 anywhere much disturbed. They appear to be steady on the cliff or 

 to roll about on the reef, but the latter is probably due to unequal deposi- 

 tion and to the washing away of the soft clays. 



( 375 ) 



