Chap. V.] trichinopoly district — ootatoor group. 71 



must have been the deeper water, at angles varying from 8° to 30°: but from its 

 partial occurrence there is good evidence to show that this structure was due to the 

 piling of the sand, which was the original material of the stone, developed by subse- 

 quent shrinking, and not to the quiet deposition on a horizontal bottom and subsequent 

 distui'bance. 



The color of the limestone is sometimes almost a pure white, and in this variety 

 p , „ ,. , patches occur of almost a chalky structure: at other times 

 it is of pale pink, yellow, or cream colored tints, the latter 

 being, apparently in some cases, due to the intermixture of fine argillaceous sediment, at 

 others to subsequent infiltration of coloring fluid. I have only found one or two 

 minute enclosed pebbles throughout the whole formation, these being of quartz or 

 gneiss, and occurring at the base of the ridges. With the exception of the case men- 

 tioned at page , I have never noticed any of the embedded masses of the lime- 

 stone itself, such as Mr. Darwin mentions as entering into the structure of the outer 



portions of recent coral formations. In composition the 

 Chemical composition. , . . , i. . mi 



rock is,^almost a pure hmestone. 1 hree specimens analysed 



by Mr. Tween, the first from Cullygoody, the second from Tripatoor, and the third 

 fi-om Olapaudy, gave only 3-4 per cent., 47 per cent., and 2 per cent, respectively 

 of insoluble matter. The soluble portions of Nos. 1 and 2 consisted of carbonate 

 of lime, with traces of iron and magnesia, and a small quantity of alkali (the 

 amount not estimated). The soluble portion of No. 3 contained 96 per cent, of 

 carbonate of lime, 2 per cent, of iron and alkali, chiefly soda. 

 Turnino- now to the consideration of recent coral formations, let us compare 

 Comparison with recent seriatim the several characters summarized above : and first 

 coral-reef limestones. ^g to chemical composition. 



An analysis of recent coral sand given by Mr. Dana* is as follows: — 



Carbonate of Lime 98-26 



Ditto Magnesia 1-38 



Alumina '24 



Si^i^^ I Traces. 



Phosphoric Acid J 



My specimens were not tested for phosphoric acid, but the quantity of carbonate of 

 lime, as contrasted with the small percentage of foreign matter they contain, Is strik- 

 ino-ly close to the composition of the recent coral sand as above given. The magnesia 

 in recent coral rock varies greatly according to Mr. Darwin ; one specimen quoted by 

 Vilm having as much as 38-07 per cent. In all cases this must be from some foreign 

 som-ce, probably, as suggested by Mr. Darwin, from sea water, and it can scarcely be 

 regarded as an essential Ingredient. 



The physical characters of the Trichinopoly rock correspond no less strikingly 



Characters of recent *<' those of recent reef-rock, as described by the same 



limestones. author. Mr. Dana says, {Op. Cit., page 110) " The coraJ- 



* Dana " On coral reefs." 



