TltANSACTIONS OF SECTION D, 645 



From off Belligam two samples were dredged, one, a flue calcareous mud with 

 shells, yielding 76-50 per cent, of carbonate of lime, and another, obtained at 

 depths of 4 to 6 fathoms, containing many rotten brown shells bored by Cliona, 

 Barnacle valves, and Halimeda in fresh condition. 



The fine material in both hauls consists of smooth mud which cakes on drying. 

 It contains very few quartz grains. Foraminifera are rare, but spicules of Sponges, 

 Ascidians, and Holothurians make up the bulk of the non-calcareous matter. 

 Diatoms, especially Coscinodiscus, are fairly abundant. 



Otf the mouth of TrincomaleeBay, at 12 fathoms, Foraminifera sand was dredged 

 containing 67"7 per cent, of one form, Heterostegina. Mollusca, including such 

 forms as Area, Trochus, Patella 4'8 pf>r cent., Corals and Polyzoa ()*2 per cent., 

 NuUipores 8'0 per cent., while Alveolina (two species) make up O'O per cent. 

 Echinocyamus occurs sparingly: IG'u per cent, passed through the fine sieve, and 

 this material consists almost entirely of Foraminifera, including Pulvinulina, 

 Textularia, Discorbina, Miliolina, and Nummulites. Pteropods, Alcyonium, and 

 Sponge spicules are fairly plentiful. The inorganic constituents include quartz 

 grains, well rounded and ranging up to 3 mm. diameter, and a black powder much 

 of which could be removed by a magnet. The non-magnetic portion was fraction- 

 ated by means of the double iodides of mercury and barium, and showed a great 

 number of garnets, corundum, tourmaline, and kyanite in the heavier fractions, 

 while quartz and a few ilakes of mica made up the lighter portions. 



North of llamiserram, in Palk Bay, where shallow water (6 to 7 fathoms) 

 conditions extend over a great area, and where there is an almost complete ab- 

 sence of currents, a very extraordinary deposit occurs : it consists mainly of con- 

 cretions, irregular in form, with here and there a cast of shell along with a few 

 large shells in a fairly fresh but broken condition. The shells include Area, Car- 

 dium, Modiola, Chama, Pecten, Murex, Nassa, and the Pearl Oyster. No double 

 valves are found and the calcite varieties are sometimes bored by Cliona. No fine 

 material accompanies the deposit. The concretions, however, on treatment with a 

 •weak acid effervesce strongly and yield a large percentage of fairly coarse sand. 

 The casts, which yield O-t-80 per cent, of carbonate of lime and 2-2 per cent, of 

 pho.sphate 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 peri- 

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

 of the shell's inner surface : this is succeeded by a darker layer, and then the whole 

 interior is seen to consist of sand grains, quartz, tourmaline, felspar, and zircon, 

 embedded in a mass of secondary calcite. The sand grains increase in size on 

 proceeding from the exterior inwards, 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 noteworthy that felspars are 

 found in these casts, while they ars absent, as a rule, from sea sands. They must 

 have been embedded very soon after their breaking away from the parent rocks and 

 before kaolinisation had reduced them to clay. 



The inorganic fragments dissolved out 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 (inclosed in garnet and 

 free), kyanite, quartz, mica, and felspar. A number of black grains were composed 

 of ilmenite. 



Farther east, and just north of Adam's Bridge, at a depth of 7 fathoms, a fine 

 black mud occurs, wmch on analysis gives : — 



Water and Organic Matter , , 

 Silica . . .... 



Carbonate of Lime .... 



Phosphate of Lime . . , , 

 Ferric Oxidp . . . , , 



Alumina 



Magnesia 



