48 Dr. Mac CuUoch on the 



sents numerous and remarkable examples of this nature : and, 

 in that neighbourhood also, are to be found masses of ordinary 

 greenstone incumbent on the most recent strata, the forms of 

 which so perfectly resemble those of granite, in the prismatic 

 division of the parts and the subsequent rounding of the angles, 

 that they are undistinguishable without manual examination. 

 The Corstorphine Hills contain the examples of this latter occur- 

 rence, as the rocks near the Queen's-ferry do those of the 

 highly crystalline texture. 



Having thus pointed out, in a general manner, the resem- 

 blances that occur between some of the rocks which belong 

 to granite and others which are members of the trap family, 

 I may notice, but in the briefest manner, the most conspicuous 

 differences which exist between them. Those of a geological 

 nature have been sufficiently described in treating of the points 

 of resemblance in this respect ;^and it only remains to notice 

 more particularly those differences in the mineral composition 

 and character which have not been so fully stated as they 

 deserve. 



In respect to the mineral ingredients, the two substances, 

 felspar and hornblende, occur abundantly in both divisions ; 

 but, in granite, quartz and mica are very common and conspi- 

 cuous, whereas, in the rocks of the trap family, they are ex- 

 ceedingly rare. In the latter, compact felspar is also a very 

 common mineral, but it occurs in granite rarely and in small 

 quantities, apparently rather as an accidental than an essential 

 substance. Augit, also, and hypersthene, which I have pointed 

 out as ingredients in some of the trap rocks, have not hitherto 

 been found in any varieties of granite. 



With regard to the several rocks of the trap family, it has 

 been a principal object of this paper, to show that greenstone, 

 basalt, and even clay stone, occurred as varieties of granite. It 

 is yet uncertain, as before remarked, whether porphyry may 

 not also, in some cases, be a member of the granite family ; 

 but, whether it be so or not, the limits between the two are 

 often very evanescent. In granite, however, no instances have 



