Ch. XV.] OF THE PRIMARY ROCKS. 351 



which are similarly situated in clay-slate, and which, as in 

 the gneiss and mica-slate, are so closely blended therewith as 

 to pass imperceptibly into each other. Sometimes, indeed, 

 these rocks are variously intersected by quartz-veins, which 

 every one would pronounce to be contemporaneous, and yet 

 these often assume a compound crystalline character, and in 

 some parts perfectly resemble the granite-veins. And it 

 must also be observed, that when granite-veins attain a foot or 

 more in thickness, the open seams traversing the containing 

 rock, dividing the mass into concretions, often pass through 

 the veins, giving rise, when the concretions are detached, to 

 fragments or blocks which consist partly of slate and partly 

 of granite : precisely as happens at the junction of the main 

 masses of granite and slate, when these have not been separ- 

 ated by seams of structure coincident with the line of union. 



In short, the granite-veins put on such an endless variety of 

 appearances, as to size, length, and number ; and sometimes 

 interlace and reticulate the schistose rock in such a com- 

 plicated manner and so profusely, that the whole mass 

 appears to be a congeries of these veins, the basis, as the 

 gneiss at Cape Wrath, forming but a small portion of the 

 whole : so that it can be compared only to the venous appear- 

 ance of a thickly mottled and variegated marble. This fact 

 alone ought to lead us to suspect, that probably these veins 

 have not resulted from the injection of granite into the 

 fissured slate. We have, however, been so long accustomed 

 to consider that all veins were once fissures, and that all 

 portions of rock that have an elongated form or structure 

 are veins, that it requires no little effort to prevent ourselves 

 from jumping immediately to this conclusion, whenever we 

 meet with such appearances. And yet we know that it has 

 been frequently stated, that even sandstones, gypsums, and 

 other sedimentary deposits often exhibit forms which, in the 

 primary rocks, would be denominated veins. Professor 

 Jameson has repeatedly pointed this out ; and Dr. Macculloch 

 has also recorded some appropriate examples. After having 



