502 OVERLYING ROCKS. 



always determine that of the mass whence it was 

 taken. Thus it passes into many other varieties, 

 as well as into the preceding ; occasionally also 

 becoming simple by the exclusion of the horn- 

 blende. This forms one of the varieties of the 

 syenite of mineralogists ; limiting that term to 

 rocks of the overlying family. 



c. The mixture imperceptible, or nearly 

 so, to the naked eye. 



In this variety, the hornblende is in equal or 

 greater proportion to the felspar, which latter is 

 generally also of a dark colour. Hence the mix- 

 ture is dark grey, or blackish green, or nearly 

 black. The substance here called compact fel- 

 spar is often, possibly, clinkstone, or indurated 

 claystone ; but the distinction can scarcely be 

 made in such circumstances. This is also enu- 

 merated among the basalts ; presenting the same 

 general appearance as some varieties of D, E, 

 and G, First Division. Like those, it is also 

 sometimes columnar, as well as laminar. It ap- 

 pears further to pass into other varieties, by a 

 change of the character of the compact felspar, 

 or by omission of one or other of the ingredients. 



