SOO Dr. Mac Colloch on the Geology of Glen Tilt, 



instances to which no distinct denomination has been applied, as 

 this term of syenite has been to those which contain hornblende. 

 And since the granite of mica, felspar, and quartz, is sometimes 

 found to contain here and there a solitary crystal of hornblende, 

 with which in a series of gradation those varieties in which horn- 

 blende at length prevails to the exclusion of the mica are connected, 

 it is impossible to define the point at which granite is to end and 

 syenite is to begin : there is no difficulty in modifying the appli- 

 cation of the adjective term. But another, and the principal ad- 

 vantage arises from the limitation of the term syenite to a rock of 

 which the mineral character is much more constant, at the same 

 time that it is in general very strongly distinguished from granite 

 in its appearance. In its geological connections also it differs so 

 widely that it is absolutely necessary to remove all chance of their 

 being confounded together, since this rock is connected with the 

 trap family, and is found in a position in which neither granite 

 nor syenitic granite has ever yet been seen, namely, incumbent 

 on the rocks of secondary formation. 



To proceed to the description of the rock at the junctions for- 

 merly described. It consists chiefly of high red felspar and white 

 quartz, of which the latter is generally in the smallest proportion, 

 the compound mass containing obscure crystals of dark green horn- 

 blende. These are seldom in large quantity, more commonly 

 they are thinly dispersed through the rock, and not seldom even in 

 the largest masses of rock they are altogether absent. I have not 

 observed mica In any of the granite at the junctions described, 

 but in several parts of the hills the same rock, or one not to be 

 distinguished from it except by th'.s circumstance, is found to con- 

 tain mica together with hornblende, which latter not unfrequently 

 becomes excluded altogether and the simplest form of granite re- 

 mains. 



