SALISBURY CRAGS. 55 



in the upper portions conspicuous. Besides the changes 

 which the stratified rocks exhibit from the agency of the 

 trap, the greenstone also evinces alterations as it approaches 

 the strata on which it rests and which surmount it : it gra- 

 dually acquires a compact structure, either passing into an 

 almost perfect basalt, or into a very compact ferruginous fel- 

 spathic greenstone. From the circumstance of finding the 

 characters of both rocks the same, whether at the top or bot- 

 tom of the greenstone mass, it is evident that the sandstone 

 and slate which lie above the greenstone, have not been 

 deposited upon it ; but have been separated from the strata 

 which lie below the greenstone, by the subsequent intrusion 

 of the trap-rock. 



The greenstone of Salisbury Crags is in general fine 

 granular, the felspar occurring of various shades of white, 

 and of brick and flesh red. Sometimes, though rarely, 

 Natrolite is associated with it, appearing to form masses 

 not of subsequent infiltration, but of an origin contem- 

 poraneous with the rock ; and very generally the compact 

 greenstone of Salisbury Crags is traversed by numerous 

 contemporaneous veins of greenstone. These veins vary in 

 the compactness of their structure, sometimes being more 

 perfectly crystallized than the containing rock, and at other 

 times almost homogeneous throughout. They run a tor- 

 tuous course through the greenstone, and are frequently of 

 great length, while the breadth is very inconsiderable, ■*- 

 perhaps not exceeding two or three inches. In structure, 

 the greenstone of Salisbury Crags exhibits nothing remark- 

 able ; it in many places has a tendency to the columnar ar- 

 rangement, and very frequently assumes a globular mode 

 of distribution, the globular concretions varying in size. 

 In the rocks of Salisbury Crags several simple minerals are 

 to be met with. Calcareous spar occurs most frequently, 

 forming either veins or imbedded masses, and is occasionally 



