SEA-BOTTOMS AND CALCRETES. 



140 



a very extraordinary deposit occurs : it consists mainly of concretions, .irregular in 

 form, with here and there a cast of shell and a few large shells in a fairly fresh hut 

 broken condition. The shells include Area, Cardium, Chama, Pecten, Murex, Nassa, 

 and the pearl oyster. No double valves are found and the calcite shells are sometimes 

 bored by Clione. The concretions on treatment with a weak acid effervesce strongly 

 and yield a large percentage of fairly coarse sand. Plate 1., tig. 1. shows shells, casts 

 and concretions from this deposit. 



The casts, which contain 64 '80 per cent, of carbonate of lime and 2"2 per cent, of 

 phosphate of lime, fall to mud when placed in water, and it was necessary to soak in 

 thin balsam and harden before a section could be obtained. Round the periphery of 

 the casts is a thin layer of calcite, which moulds itself into the inequalities of the 

 shell's interior surface ; this is succeeded by 

 a darker layer, and then the whole interior is 

 seen to consist of sand grains, quartz, tour- 

 maline, felspar, and zircon embedded in a 

 mass of secondary calcite. The sand grains 

 increase in size <>n proceeding from the 

 exterior inwards (tig. I), and remind one of 

 the well-known fact that when grains of 

 different dimensions are shaken in a basin, 

 the finer material sinks to the bottom and 

 the coarser rises to the surface. It is 

 probable, then, that the grains were rocked 

 to and fro when in a loose condition inside 

 the shell, and the cementing took place 

 subsequently. Afterwards, owing to altered 

 conditions, the outer shell was dissolved and 

 the cast left. It is impossible, owing to the 

 rolled and imperfect condition of the casts, to 



tell what shells formerly held the casts, but most of them have a form not unlike 

 Natica. It is noteworthy that felspars are found in these casts, while they are 

 absent, as a rule, from sea sands. They were probably embedded soon after breaking 

 away from the parent rocks and before kaolinisation could reduce them to clay. 



The inoreanic material dissolved out from the concretions by acids and fractionated 

 showed a great preponderance of garnets. The heavier portions were pink in colour 

 on this account. Other minerals found were corundum, tourmaline, zircon (enclosed 

 in garnet and free), kyanite, quartz, mica (biotite), and felspar. A number of black 

 grains were composed of ibnenite. 



Further East and North of Adam's Bridge, at a depth of 7 fathoms, a tine Mack mud 

 occurs which, on analysis, gives: 



Fig. 1. Section of internal cast of shell from 

 Palk Lay : showing the part in contact with 

 the shell, and sand grains cemented by car- 



bonate of lime in the interior. 



x 2D. 



