36 Dr. Mac CuUoch on the 



points, pervades all the unstratified rocks, however' distant in 

 position and in apparent antiquity. 



Quartz so rarely enters as an ingredient into these substances, 

 that it may be altogether excluded from the present consider- 

 ation. The fundamental composition consists of felspar and 

 hornblende ; and, according to the magnitude of the parts, and 

 the relative proportions of these ingredients, the appearances 

 of the specimens vary. In some rare instances, the crystals of 

 hornblende are so large as to attain half an inch in length, 

 although they are not defined in form ; and as the felspar is 

 commonly white, these varieties form beautiful specimens for 

 collectors of rocks. From this size, the portions of each mi- 

 neral vary in gradation ; forming compounds which are undis- 

 tinguishable in every respect from the coarser and finer green- 

 stones of the trap family; from those, at least, in which 

 common, and not compact felspar, forms the other ingredient in 

 union with the hornblende. 



In all the cases which came under my notice, the hornblende 

 is invariably black, but it is not always intermixed in an uniform 

 manner with the felspar; some instances occurring in which, 

 to the general indiscriminate mixture, are superadded large and 

 distinct patches or irregular crystals ; producing that appear- 

 ance which, when it takes place in ordinary granite from a 

 similar disposition in the felspar, has been called porphyritic. 

 In general, the felspar is white, and of that variety which is 

 called common. But in the minuter states of intermixture, it 

 has often a greenish hue, and so far loses its crystalline ap- 

 pearance, as to resemble the ordinary compact felspar which is 

 more common in the greenstones of the trap family than the 

 crystallized kind. Whether these varieties, however, actually 

 contain compact felspar, I have not quite satisfied myself; and 

 the confusion which sometimes exists between these two mi- 

 nerals is such, that I am willing to leave this point undeter- 

 mined ; however inclined to believe, that compact felspar 

 occurs in these rocks just as it does in the greenstones of the 

 trap family. 



