370 



TllANSACTIOWS OF THE 



to true veins than any others ; hut they have been in- 

 eluded in this order, as opinions have been entertained, 

 that they bear the same relation to serpentine that kaohn 

 bears to granite. Veins of white steatite have also been 

 observed in clay slate. 



Veins of calcareous spar have been found in other for- 

 mations than the limestone, and in such cases come under 

 the present order. They have been found in the serpen- 

 tine, clay slate and grauwacke. In the serpentine and 

 grauwacke they are generally small, short and tortuous, 

 but have distinct walls. In the slate, althcagh also small, 

 they are for the most part tolerably straight, and have 

 regular walls, 



Elvan courses, more generally considered as porphyry 

 dikes, are composed of a mixture of hornstone, (|uartz 

 and feldspar, having the appearance of hornstone por- 

 phyry. Other substances are, however, termed elvan 

 by miners. A stone composed of very compact horn- 

 blende and chlorite, is called blue elvan; a mixture of 

 hard hornblende and quartz has received the same name; 

 a compound of feldspar and hornblende is also called 

 elvan ; a combination of hornstone quartz, schorl and 

 chlorite, has also been thus designated ; and fine grained 

 granite has been thus called. Hardness does not appear 

 to be an essential quality of elvan. 



The elvan courses vary in width from 1 to 60 fathoms. 

 Their direction is generally a little north of east and 

 south of west, and they almost always underlie toward the 

 north, at an angle of 45°. The extent of their length has 

 never been ascertained, although one has been traced as 

 far as 5 miles. Elvan courses seldom, almost never, in- 

 tersect true veins of copper or tin, although they fre- 

 quently intersect and heave lead lodes. 



Elvan courses have been considered as contemporane- 

 ous with their inclosing rocks, because they are traversed 



