Chap. VI.] trichinopoly district — ootatoor group. 101 



of the deposit is the change in that of the fauna, and while Cepha- 

 lopoda increase greatly in the number and 

 Fauna. n -i • n 



variety of their forms, they are intermingled in all 



proportions with Gasteropoda and Conchifera, drifted together in 

 immense numbers. In the coarsest beds, indeed those in which peb- 

 bles of gneiss are abundant, fossils are comparatively rare. Beds 

 of moderately fine sand or argillaceous sand and limestone (originally 

 calcareous muds) are on the whole the most fossiliferous. In these 

 beds, then, we have evidence of a very different state of things 

 to that which prevailed to the Southwards, The materials of the 

 Northern deposits were brought down by currents, bringing, together 



Mode of accumulation ^'^^^ ^^^ «'^°^ ^^^ argillaceous matter which 

 of deposits. form the bulk of the deposit, large numbers of 



shells from the bottom over which it passed, and occasionally pebbles 

 of gneiss and limestone which were drifted along the bottom to a distance 

 of some miles. These coarser materials, rare at first, became more and 

 more abundant, as, with the accumulation of deposits, the sea became 

 shallower, and the transporting power of the current consequently 

 stronger, and encroached continually on the area to the Southward to 

 which fine sediment alone had been previously carried. That Cephalo- 

 podous forms, especially ground feeders (tebrabranchiata) should be more 

 abundant here than to the Southward is only what we might expect 

 from a consideration of their habits, as they would naturally most 

 abound (cceteris paribus) where those animals were most abundant 

 upon which they depended for a supply of food. 



These deductions being accepted, we have next to consider from which 

 Indications afforded by ^^i^ection the materials of our deposits were derived, 

 the bedding. ^^^ ^^^ ^j^-^ p^^j-pose we must first summarise the 



facts of the bedding, and see how far this structure has been influenced 

 by the operation of marine currents. We have to consider whether the 

 phenomena observed are best explained by original irregular deposition. 



