336 GEOLOGICAL APPEARANCES 



stance is often quite the same with that of the main bodv of 

 the sienite, but they also exhibit a variety of gradations from 

 it. The crystallised grains are often larger in the veins ; in 

 some of them there is a little mica; and sometimes, though 

 rarely, they contain white felspar mixed with the red. 1 here 

 are small veins of red felspar and hornblende, running parallel 

 to the stratification of the hornblende-slate, and veins of the 

 same sort occasionally traverse the strata* The lines of junc- 

 tion between the veins and the hornblende-slate are generally- 

 well defined, and often extremely so ; sometimes, however, the 

 substances are blended. I imagine that, by a nice selection of 

 small specimens, a series of gradations might be made out be- 

 tween the hornblende-slate and the sienite ; but in the general 

 aspect of the two substances, even where in close contact, 

 there is a marked distinction of character. The veins are 

 sometimes reticulated, and display the greatest irregularity in 

 their outlines and direction. I believe that those parts of the 

 veins, which are near the main body of the sienite, may in 

 almost every instance be traced to be continuous with it. 



88. The southern abutment of the bridge rests on a rock, 

 which projects considerably under the arch. The larger part 

 of it is formed of the main body of the sienite, but it contains 

 many masses of hornblende-slate, in which felspar is some- 

 times interstratified. A large one, on the east side of the rock, 

 contains much felspar, and is penetrated with carbonate of 

 lime. Of the larger masses, some cannot be traced to rest en- 

 tirely upon the sienite ; but others can, and some of the small- 

 er, which dip into the face of the rock, are manifestly imbed- 

 ded in the sienite. There are adjacent masses, in which a cor- 

 respondence of form argues that they were once connected. 

 The position of the masses, with regard to their stratification, 

 is very irregular; and as instances of it I may mention, that on 



the 



