250 



VEINS AND VEIN-STONES. 



horizontal, but having a slight dip to the east ; it increases in 

 hardness near the junction. The granite, which is generally coarse 

 and porphyritic from the large embedded crystals of felspar, becomes 

 here of a finer grain, with black mica and light flesh-red felspar. 

 On the north it laps over the schist. At this spot numerous granite 

 veins, varying in width from about a foot to less than an inch, pass 

 through the slate ; the two principal veins proceed nearly east from 

 the hill above, for more than fifty yards, until they are lost in the 

 sea. One of these, not far from its first appearance, is divided and 

 heaved several feet by a cross vein consisting of quartz intermingled 

 svith slate ; fragments of slate appear also in the granite veins. The 

 most remarkable vein, after proceeding vertically for some distance, 

 suddenly forms an angle, and continues in a direction nearly hori- 

 zontal, having slate above and below." 1 



The Devonian rock, locally called killas at this place, has much 

 the aspect of greenstone, and it appears generally true that the clay- 

 . / slate is much altered in character round 

 all the granites of Cornwall and Devon. 2 

 The veins of granite are generally most 

 fine-grained towards the walls. Von 

 Oeynhausen and Von Dechen mention 

 three principal veins at Mousehole, one 

 3j to 10 feet wide; quartz veins cross 

 the direction of the granite veins, and 

 sometimes divide them, and apparently 

 alter their character. Schorl occurs ir- 

 regularly in the granite, and in some of 

 the quartz veins. The intricate character 

 of the veined masses of Mousehole will 

 be best understood by consulting the dia- 

 gram (fig. 58), copied from the sketch of 

 the distinguished Prussian geologist above 

 named. 



s 8 - At Cape Cornwall a granite vein heaves 



a quartz vein in a direction contrary to the general law. In the 

 Lizard district granite veins divide serpentine. 



Veinstones. Under this name Dr. Sterry Hunt, F.R.S., designates 

 a class of veins which are neither injected from below nor infiltrated 

 from above, but derived from the rock which immediately surrounds a 

 closed fissure in which water accumulated. This origin of felspathic 

 and granitic veins by segregation was first indicated by Daubree, and 

 subsequently taught by Dana. In schists in the Appalachian region 

 of Canada are veins with flesh-red orthoclase mixed with chlorite 

 and white quartz. Some veins are made up of orthoclase, quartz, and 

 mica ; they have the character of coarse granite, and include a number 

 of rare minerals in small quantity. Granitic veinstones abound in 

 the Montalban series in the United States. Sometimes the felspar 



1 Cornwall Geol. Soc. Trans., vol. i. 



2 J. A. Phillips " On so-called Greenstones of Cornwall,'' Q. J. G. S., vol. xxxii. 



