SKY. GEOLOGY. OVERLYING ROCKS. 371 



the other ingredient: the porphyritic character is some- 

 times superadded to this mixture ; while in some rare 

 instances quartz enters into the composition, and it then 

 borders upon syenitic granite. More rarely still it contains 

 mica, and in this case it cannot be distinguished from 

 those granites which contain crystals of hornblende super- 

 added to the usual threefold mixture of quartz, felspar 

 and mica. Under such circumstances it is conceivable 

 that specimens should exist without hornblende, since 

 even in those I have described, it is sometimes very 

 thinly scattered through the mass. In such a case, should 

 it occur, mineralogy, unassisted by geological observa- 

 tion, would tend to mislead us respecting its position; 

 and we are thus compelled to acknowledge, in geological 

 description, the necessity of superadding to mineral cha- 

 racters an accurate knowledge of the connexions of the 

 rocks respecting which we are reasoning.* It is there- 

 fore from a geological knowledge of the position of the 

 present rocks, that they are referred to the syenite divi- 

 sion, since had the same specimens occurred in a mountain 

 of granite and lying under micaceous schist, they would 

 have been ranked with the granites. f 



* This is far from being the only case in nature where mere mineral 

 distinctions are insufficient to determine the geological situation of a 

 rock. In the stratified rocks, both primary and secondary, these resem- 

 blances are frequent; since it is often impossible to distinguish quartz 

 rock from sandstone, the breccias which it Contains from the more recent 

 graywacke, ancient clay slate from recent, or, as I have shown in this 

 account of Sky, primary from secondary limestones. The same 

 rocks seem in some cases to have been repeated at different epochas, 

 while in others they show variations which may perhaps be the results of 

 posterior changes operating on the first deposits, rather than the conse- 

 quences of original differences. 



f The character of this syenite gives rise to some conclusions 

 that are not unimportant. At present, it is easily mistaken in 

 hand specimens for a variety of those granites which are entirely sub- 

 jacent to the older rocks and divested of any pretensions to the over- 

 lying character. With a very slight change of composition it could not 

 be distinguished. That such a change occurs in other situations seeaas 



