OF FASSNEY. 



129 



a disintegrated rock, composed of felspar, horn- 

 blende, and mica 5 and these continue, with little 

 interruption, for nearly a mile down the brook, a 

 little to the eastward of Mill Know, where the 

 grey wacke again comes in, and upon which they 

 rest. 



The uncommon thickness of the beds of this 

 beautiful rock, and the covered state of;the coun- 

 try, rendered it very difficult to ascertain its geo- 

 gnostic relations ; but I soon satisfied myself, by at- 

 tending to the course of the water, that the thick- 

 ness is in a great measure only apparent, and that 

 the same strata were cut by the water three times 

 in pursuing a very serpentine course. This de- 

 ception is easily made apparent, by drawing lines 

 through the course of the stream, as delineated on 

 the map. Small veins of heavy- spar, from one 

 inch to four in thickness, occur in this rock. As 

 this rock contains mica in addition to felspar, it 

 might be supposed, by those who only attended in 

 a superficial manner to oryctognosy, to be essen- 

 tially different from either Sienite or Greenstone, 

 and to deserve a particular name. But a number 

 of reasons here lead us to conclude, that the mica 

 is an accidental or adventitious ingredient, similar 

 to crystals of felspar in basalt, or garnets in 

 mica-slate, by which the general characters of 

 these rocks, or their names, are not affected. A 

 specimen taken at a little distance from the water, 

 favours the supposition ; it approaches to common 

 greenstone, by the felspar assuming a white co~ 



1 



