490 



J. A. PHILLIPS ON THE SO-CALLED "GREENSTONES' 



by the formation of a dark grey dusty product of alteration which is 

 almost always present in modified doleritic rocks. In some places 

 the plagioclase has become merged into a granular mass, containing 

 patches of calcite, viridite, and hornblende, and enclosing a large 

 amount of altered ilmenite, with numerous crystals of apatite. 



Near Carkeel greenstone is obtained from a quarry situated at a 

 short distance north-east of the road ; but this rock has undergone 

 such extensive alteration as to afford but little information relative 

 to its original composition; in its present state it consists of a 

 granular mass, in which numerous indistinct microlites cross one 

 another in all directions. These minute crystalline bodies, however, 

 are rendered opaque by hydrated ferric oxide, the only recognizable 

 minerals being viridite and magnetite. 



At Cumble Tor, on the left bank of the river ISTottar, nearly oppo- 

 site Holwood, a quarry is extensively worked upon a fine-grained 

 dolerite, in which the felspar is to a large extent unaltered. The 

 augite, which is not abundant in this rock, is sometimes stained by 

 oxide of iron, while the titaniferous magnetite is generally replaced 

 by the usual light-coloured substance. 



This rock is overlain by an ancient lava-flow, which, however, is 

 separated from it by a few feet of slate, the character of which has 

 become considerably modified by the action of the influences to which 

 it has been exposed 



The accompanying woodcut will afford an idea of the appearance 

 of the section exposed in Cumble-Tor Quarry, in which a represents 

 slate, b the lava-flow, and c dolerite. 



Section exposed in Cumble-Tor Quarry. 



a. Slate. 



b. Lava. 



c. Dolerite. 



The lava b is vesicular, and contains a large amount of compara- 

 tively unaltered plagioclase, but no augite, which has apparently 

 become replaced by viridite, and by the flocculent dusty-grey sub- 

 stance which in the rock from Hatt is seen to result from the de- 

 composition of that mineral. 



In this quarry there are patches of rock which have become decom- 



