72 Dr. Mac Culloch's Sketch of the 



hardened it is known by the name of compact felspar. As specimens 

 occur in this simple state they must be considered mineralogically as 

 examples of these different substances, although in a geological sense 

 we cannot without troublesome circumlocution and great confusion 

 consider them under any but the general term already adopted. The 

 colour of this base varies from ochrey yellow and dirty flesh colour 

 to gray : it is often cavernous and filled with a ferruginous clay. In 

 other situations it contains crystals of felspar, either of the same or 

 of a different colour, and thus forms various kinds of porphyry. 

 The predominant form however is that whence its name has been 

 imposed, an aggregate of felspar and hornblende, in which the 

 hornblende generally bears a very small proportion to the other 

 ingredient : the porphyritic character is sometimes added to this 

 mixture. In some rare instances quartz enters into its composition, 

 and in such instances it trenches near upon the syenitic granite, a 

 distinction concerning which I have spoken in the paper on Glen 

 Tilt, to be found in this volume. More rarely still it contains mica, 

 and in this case it becomes utterly impossible to distinguish it from 

 those granites which contain crystals of hornblende superadded to 

 the usual threefold mixture of quartz, felspar and mica. Under 

 such circumstances it is quite conceivable that specimens should be 

 met with from which the hornblende was absent, since even in 

 those I have described, it is very thinly scattered through the mass. 

 In such a case, should it occur, mineralogy, unassisted by geological 

 observation, would tend to mislead us in reasoning respecting its 

 position, and we are thus driven to acknowledge, in geological 

 description, the necessity of superadding to mineral characters an 

 accurate knowledge of the connections of a rock respecting which 

 we are reasoning : I must therefore, from a geological knowledge of 



