346 OKIGIN- AND DEVELOPMENT OP ORE-DEPOSITS. 



locally silicified so as to furnish very fair oil-stones, hone- 

 stones, scythe-stones.* 



The siliceous alteration of limestones is a not uncommon 

 occurrence in East Cornwall and West Devon, but in these cases 

 the cherty bands and the silicified fossils have more of a 

 non-crystalline or flinty character than is observable in the quartz 

 veins and siliceous capels of the West of Cornwall. It should 

 be remarked, however, that the quartz in the north and south 

 lead-bearing lodes of Menheniot and St. Pinnoek (Wheal Tre- 

 lawney, Wheal Mary, Wheal Ludcott) is largely of a chalcedonic 

 character, and so also is that of certain north and south veins 

 near Withiel. This infiltration, with chalcedonic rather than 

 crystalline quartz, is also met with in some of the fine-grained 

 elvans such as those of Trelaver Downs in St. Dennis and 

 Foxhole in St. Stephens. f 



Chalcedonic quartz is not, however, limited to Devon and 

 East Cornwall, since much chalcedony has been found in the lodes 

 at Trevascus, Pednandrea, and many other mines between 

 Pedruth and Marazion, and also at Wheal Pose and Wheal 

 Penrose, near Helston. 



At St. Just opaline silica has occurred in many different 

 forms and in considerable abundance at Botallack, Wheal Cock, 

 and other mines. 



The silicified sandstones of the greensand beds at Blackdown 

 are often more or less concretionary and chalcedonic, while at 

 Lyme Regis beds of sand have been extensively infiltrated with 

 chalcedonic quartz. 



Thus then, in the West of England, we have silica existing 

 in secondary deposits in at least three distinct forms ; each 

 exhibiting marked modifications, as follows : — 



(a.) Crystalline, as clear or tinted rock-crystal, as variously 

 coloured and ordinary vein-quartz, as "fibrous" cross-course 

 spar, as "sugary spar," " floatstone," &c., all practically 

 anhydrous, and with specific gravity of 2 "6 and upwards. 



* The famous Water of Ayr Stone is a silicified rock of this character. 



t The super silication of felsites and felspar porphyry s is very general in the 

 Eio Tinto District (Q. J.G.8., 1885), and also in the rocks of Seville (Macpher-: 

 son, Bol. de la. Com. del Mapa. Geol., Tomo VI.) 



