LIMESTONES OP SOUTH DEVON. 



381 



rocks frequently cut through them, and beds of volcanic ash are 

 interstratitied with them." ^ There is evidence which seems to in- 

 dicate that the molecular changes (crystallization) which have taken 

 place in these limestones were influenced by, if not directly due to, 

 the disturbances mentioned by Mr. Ussher. 



This observation leads up to another very interesting question, 

 namely, the origin of the rhombohedral crystals of dolomite. Though 

 I have so far only made a passing mention of these crystals in one 

 or two slides, yet they are to be found in most of those that I have 

 examined, more especially in ]S"o. 3 from the Barton Limestone (see 

 the accompanying figure). Sometimes they appear in aggregates, at. 

 other times as single crystals, and there is no doubt that they are 

 of secondary origin. 



Crystals of Dolomite in Devonian Limestone. 



XSOdiam. 



[Since this paper was written I have learnt that attention had ah-eady been 

 called to the occurrence of dolomite crystals in Devonian Limestone by 

 Dr. Sorby, F.E.S., Quart. Journ. Geol. Soc. vol. xxxv. (1879) Froc. p. 89.] 



Several theories have been advanced to account for the formation 

 of dolomite, and they may be all quite correct, for I see no reason 

 why this mineral should not originate as the result of more than 

 one process ; as, for instance, the percolation of sea-water through 

 limestone. The Devonian Limestones, however, are only par- 

 tially dolomitized, and I think it possible that the magnesia may 

 have come from the decomposition of magnesian silicates deposited 

 as detritus with the limestone. In support o£ this hypothesis it 

 may be mentioned that dolomite is sometimes present as one of 



1 Quart. Journ. Geol. Soc, vol. xlvi. (1890) p. 487. 

 Q.J.G.S. :t^o. 19L 2e 



