A. K. Coomdraswdmy — Crystalline Limestones of Ceylon. 375 



Wash, as was suggested, I think, by Prestwich; that would be 

 equivalent to expecting a river to run along the level instead of 

 down the slope ; nor could the Upper Severn be given an original 

 course into the Dee, suggested by Professor Lapworth, that would 

 be almost equivalent to asking it to run uphill. 



Eivers may abandon the hard course for the easier one, but not 

 the easy one for the difficult. If the Thames had originally gained 

 the sea by the Wash, and the Upper Severn got to the Dee, some 

 particular damming- up theory has to be put forward to account 

 for their leaving these easy courses to cut through the Chalk or 

 Silurian barriers respectively. That is to say, abnormal conditions 

 have to be postulated to explain a common and normal phenomenon. 

 And when these conditions are applied to other cases they fail. 

 Applied to the Weald they would mean that the drainage originally 

 escaped via Hastings, then that this outlet was blocked in such 

 a peculiar manner as to cause the drainage to cut several separate 

 and independent channels through the Chalk to the north and 

 south of the Weald ! 



I 



VII. — Origin of the Crystalline Limestones of Ceylon. 



By A. K. CooMARAswAMY, B.Sc, F.L.S., F.G.S. 



T is generally supposed that the crystalline limestones which 

 occur amongst the schists, associated with orthogneisses, in 

 various parts of the world, are altered sedimentary limestones whose 

 accessory minerals and crystalline structure have been developed by 

 simple contact and dynamo-metamorphism ; and no doubt many 

 crystalline limestones are of sedimentary origin, and owe their 

 peculiarities to these agencies. In other cases the peculiar mode of 

 occurrence of such rocks or their manner of association with igneous 

 rocks (orthogneisses or nepheline-syenites) has led to the suggestion 

 of otber theories. 



Professor Hogbom ' has described the crystalline limestone which 

 is associated with the elseolite-syenite of Alno, where all transitions 

 occur from pure limestone to calciferous and normal elseolite-syenite. 

 Intergrowths of calcite with nepheline, eegirine, and felspar were 

 noted. Professor Hogbom (p. 109) thinks that, whatever the origin 

 of the limestone (it seems not likely to be an altered sedimentary 

 one), it is quite certain that it has been melted and taken up by the 

 magma on a large scale without decomposition, and that on con- 

 solidation, calcite has crystallized out of the magma in just the 

 same way as the other minerals. 



Professor Judd^ has suggested that the crystalline limestones of 

 Burmah (which in many respects very closely resemble those of 

 Ceylon) result from the alteration of the basic lime-silicates of the 

 pyroxene-gneisses. My observations show that this is a theory not 

 applicable to the crystalline limestones of Ceylon. This is shown 

 by the field relations and by the fact that the Ceylon granulites are 



1 Geol. Foren. Stockholm. Forh., 1895, vol. xvii, pp. 100 and 214. 



2 Phil. Trans. Roy. Soc, 1896, vol. clxxxvii A, pp. 151-228. 



