DEVONIAN SYSTEM : PLATEAU LIMESTONE. 



189 



A. very significant fact brought out by these analyses is the com- 

 paratively small proportion of insoluble resi- 



Ongin of the dolo- ?r f 1 l . . 



m ite S . o-ue that they contain, except in those cases 



where the high percentage of foreign material 

 is obviously due to the proximity, during the period of deposition, 

 of argillaceous or siliceous rocks. Prof. Skeats has shown, in his 

 researches into the composition and history of the coral-reef de- 

 posits of the South Seas, 1 that an almost inappreciable quantity of 

 insoluble residue is a characteristic feature of limestones that have 

 been formed under coral-reef conditions ; and in applying the 

 result of these researches to an enquiry into the origin of the 

 dolomites of the Southern Tyrol, 2 he employs this characteristic 

 as an argument in favour of the hypothesis that those rocks were 

 formed under such conditions. In other respects also, in the 

 manner in which dolomitisation has affected the rocks, in their 

 composition and mode of occurrence, the Plateau Limestones re- 

 semble very closely, so far as one can judge by the descriptions 

 published, the dolomites of the Tyrol, and I think that it is reason- 

 able tj ascribe to them a similar mode of origin. 



Thin sections of the specimens, the analyses of which are given 



in Table 7, as well as of a large number of 

 Petrographic characters. . . , , , . . 



other specimens, not analysed chemically, have 



been examined under the microscope. In all cases where more than 

 10 per cent, of magnesium carbonate was shown to be present by 

 ( ( .1 analysis the sections were also tested by 



Lemberg's solution, prepared according to the 

 instructions given by Prof. Skeats in the first of his papers cited 

 above. 3 but of only half the strength recommended by him ; as it 

 was found that, probably on account of the higher tempera- 

 ture prevailing in Calcutta, the action of the solution was verv 

 rapid, and it was difficult to avoid the deposition of a thick layer 

 of alumina on the calcite, with consequent shrinkage on diving. 



1 Chemical Composition of Limestones from upraised Coral Islands ; Hull. Mus. 

 Comp. Zooloyy, Vol. XL1I, p. 103. 



2 Quart. Journ. Qtol. Soc. Loudon, Vol. LXI, p. 97. 



3 The formula of this solution, ns </iven by I'rof. Skeats, is as follows :■ — 



Aluminium chloride, dry, I parts. 

 Water, 00 parts. 

 Dissolve, then add :— 



Hcematozylin cavtpechicmwn (Logwood), fi parts. 

 Boll ;md stir for minutes; filter, and make up to the same bulk with water. 



