344 OBIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 



parts of Devonsliire ; a similar phenomenon is observable in the 

 Perran and Mithian sands, and especially in the sands near 

 Crantock and St. Enodoek. The extensive dunes of blown sand 

 along the north coast contain a large proportion of carbonate of 

 lime in the form of comminuted shells. The rain-water in 

 sinking through the upper layers of the sand dissolves out part 

 of the carbonate of lime, and in favourable circumstances, where 

 there is an underlying stratum of comparatively impermeable 

 ground, this is re-deposited in such a way as to convert the 

 lower layers of the sand into a fairly compact and durable sand- 

 stone of great value for (local) building purposes. 



On the other hand, the withdrawal of carbonate of lime in 

 solution has frequently altogether removed or extensively 

 changed beds of calcareous rock, as in the case of the coral- 

 bearing shale at Newham alluded to. Here the forms of the 

 corals and even their minute structure are still perfectly distinct, 

 yet the rock now contains a mere trace of lime.* There is no 

 means of knowing where the lime has gone in this instance, but 

 in general the lime must have been removed before the principal 

 lodes of the West of Cornwall were formed for it is an undoubted 

 fact that with a few local exceptions the lodes in this part of 

 the county, where limestones are almost unknown, contain very 

 little calcite as a veinstone. 



Silicification, I am not aware of any notable siliceous 

 springs in Cornwall or Devon, but the mine waters analysed 

 have contained on an average nearly two grains of silica per 

 gallon, whether thermal or phreatic. Water containing this 

 comparatively small quantity of silica would still be capable of 

 silicifying shales and sandstones (as at the Haytor Mines), of 

 depositing cross-course spar (asin the numerous cross- courses), of 

 filling cracks in the rock (as seen in the killas of so many places), 

 of indurating sandstones and conglomerates (as at Ladock and 

 the Nare Point), or of forming siliceous bands in the granite or 

 siliceous capels in the killas. The process of kaolinization 

 liberates a considerable proportion of uncombined silica, and 

 this we constantly see in bands traversing the carclazyte, accom- 

 panied in some instances by schorl and cassiterite. 



^Recent Mineralogical Analyses, by J. H. Collins, Jour. B.I.C., xxiii. 



