458 Scientific Geology. 



a- 



Veins in Sienite : Whately. 



The above sketch represents an area about 10 feet long and, 6 feet 

 wide, aa, is a fine grained granite vein an inch wide at the upper 

 end, and decreasing downward: b, another granite vein of fine grain, 

 one foot wide: c, a similar one of equal width: d is a fissure, or rather 

 an epidote vein. 



aa, is obviously the oldest vein; for it is cut off by b, and this again 

 by c. Hence we have here granite and sienite of four epochs: 1st the 

 rock of sienite itself: 2d the vein a: 3d the vein b, protruded subse- 

 quently : and 4th the vein c, injected last of all. At what epoch the 

 vein d cut off a, a, we have no means of ascertaining ; only that it was 

 previous to the formation of the vein c, since this is not affected by the 

 lateral slide apparent in a, a. 



Macculloch says that " there have been granites found where a 

 vein of a third granite traversed a vein of a second and different one, 

 which had previously traversed the mass of a first."* The preced- 

 ing sketch proves a fourth production of granite : for there can be 

 no doubt but the sienite of Whately is a variety of granite. But if 

 the suggestion above made, relating to the successive production of 

 different granite veins, while yet the rock which they traverse was in 

 a fluid or semi-fluid state, be admitted; we need not suppose any long 

 interval between the formation of these veins. However, I do not 

 place confidence enough in the suggestion to make it the foundation 

 of any important inference. 



The rock in the following sketch is about 15 feet long, d, d, d,is 

 a vein of feldspar, two inches wide at its western extremity : but it 

 ramifies at the other end and almost disappears, a, b, c, are veins of 

 epidote, on each side of which the sienite has become indurated so as 



* System of Geolog-y. Vol. 1. p. 503. 



