122 ORIGIN AND DEVELOPMENT OF ORE DEPOSITS. 



situated " cutters" or " ends," (vertical joints running at right 

 angles to the backs), the granite quarryman is in his element. 

 Many of the granite quarries of the West of England are 

 especially notable for the large rectangularly jointed masses 

 which they furnish. Each of the more important quarries 

 affords examples of natural bed-joints, approximately plane 

 surfaces, extending over many square yards, and when, as is 

 often the case, the great masses so separated into beds are not 

 broken up by too numerous end-joints, the material is enormously 

 increased in value. It would be easy to supply, from the more 

 important quarries, monoliths from 12 to 20 feet long and 3 or 

 4 feet wide and thick. Fine examples of such regular jointing 

 may be seen in several of the granite quarries in the parish of 

 Mabe and Constantine, near Helston ; in the Cam Grey quarry, 

 near St. Austell ; and, indeed, in most of the larger granite 

 quarries of the West of England. 



We may be sure that the frequent occurrence of such masses 

 on the tops and sides of the hills has greatly encouraged and 

 facilitated the monumental efforts of the early inhabitants in the 

 erection of the numerous menhirs, circles, and cromlechs which 

 characterize the granite districts of the west.f 



In elvans (felspar-porphyries and felsites) the joints are 

 generally more numerous and more irregular than in granite. 

 Many elvans are divided by jointing into small angular masses 

 throughout, while others, as in the case of the fine-grained felsite 

 at Foxhole in St. Stephens, are full of curved joints. Really 

 good blocks of building stone, derived from elvan, are compara- 

 tively rare, yet the elvans at Seveock, Newham, and Pentewan, 

 and many others are capable of yielding fine and large blocks of 

 very solid stone, and many gate-posts hewn from the Seveock 

 Elvan, as much as 10 or 12 feet long without a visible flaw may 

 be seen in the neighbourhood of Truro and Chacewater. 



Joints in stratified rocks. In slates the joints are sometimes 

 far from numerous, and when the slate is fine-grained and hard, 

 and has a good cleavage, as at Tintagel, it becomes very valuable. 



*See Carne, Trans. Roy. Geol. Soc. Corn., Ill, p. 209. 



fThis natural jointing, although often related to the "quarry-cleavage" 

 referred to below is yet quite distinct in character and origin from that cleavage. 



