AUGITiC ROCKS. 41 



of the felspar ; this, however, is far from universal, for the 

 ferruginous matter which colours much of the greenstone 

 of East Lothian, is equably distributed throughout the 

 augite and felspar. In size, the components of the green- 

 stone vary from that minuteness which causes the rock to 

 become basaltic, to the large granular, when it becomes 

 Syenitic Greenstone. Besides appearing as syenitic and com- 

 pact, greenstone occurs under several other aspects. Not un- 

 frequently it assumes the porphyritic structure,- the basis be- 

 inga very minute and small granular compound of augite and 

 felspar; while the crystals which produce the structure are of 

 compact, glassy, or common felspar. By containing minerals, 

 which fill or line cavities, greenstone becomes amygdaloi- 

 dal ; this structure appears, however, generally to attend 

 those greenstones which are not highly crystallized, but 

 whose components approach to an earthy state. When 

 structure on the great scale is observable in the greenstone of 

 the Lothians, it is of two kinds, the globular and columnar. 

 Of the former there are many examples, and the globular 

 concretions vary in diameter from one or two inches to se- 

 veral feet. The perfect columnar mode of arrangement is 

 of less frequent occurrence, for, though there is, in many 

 places, a disposition to be columnar, there are but few points 

 where this arrangement is perfect ; when, however, the co- 

 lumns are well formed, they vary in length, thickness, num- 

 ber of their sides, and position. 



Basalt, which is another conspicuous member of the 

 trap series, forms considerable masses in the Lothians ; it is 

 generally of a grey or black colour ; and, as in green- 

 stone, mere ocular inspection is sufficient to prove that 

 it is a fine aggregate of crystalline masses of Augite and 

 Felspar. Olivine abounds in some basalts, forming mi- 

 nute grains ; and magnetic iron ore is also very generally 



