"GREENSTONES" OF WESTERN CORNWALL. 177 



composition, but also that in this respect they do not materially 

 differ from the highly crystalline augitic rocks of the neighbourhood 

 of Penzance. It is also remarkable that in all these rocks the pro- 

 portion of silica is very nearly constant. The greenstones, how- 

 ever, which occur between these limits afford distinct indications of 

 slaty cleavage ; and thin sections do not exhibit the distinctive 

 angular arrangement of crystals commonly seen in rocks of known 

 igneous origin. 



In this latter respect they essentially differ from a greenstone 

 quarried in a field known as the Sanctuaries, a little west of the 

 town of St. Austell, where the central portions of the mass enclose 

 felspathic and other crystals, arranged in such a way as to indicate 

 the probable igneous origin of the rock. Although the central 

 portions of this dyke are crystalline, it gradually becomes slaty on 

 either side: and it is consequently difficult to trace a precise line of 

 demarcation between it and the enclosing killas*. 



With regard to the blue elvans found between St. Erth and 

 St. Stephens, it may be remarked that their texture is frequently 

 coarser in the centre than towards the sides of the bands, and 

 that they eventually merge into the enclosing rocks, of which they 

 gradually assume the structure and cleavage. Their ultimate che- 

 mical composition agrees closely with that of the crystalline au- 

 gitic rocks before described ; but their structure is invariably slaty, 

 and thin sections do not exhibit any of the characteristics peculiar 

 to igneous rocks. It follows that if these were originally igneous 

 rocks other than ash-beds, they must have been subjected to the 

 action of influences by which the arrangement of their constituent 

 minerals has been completely changed and slaty cleavage produced. 

 They are met with only in the neighbourhood of the granite, or in 

 the vicinity of elvan courses ; and when describing those in the dis- 

 trict around Camborne, Messrs. Von Oeynhausen and Yon Dechen 

 state that they have not the appearance of regular dykes or veins 

 enclosed in clay-slatesf. The greenstones occurring in the killas 

 surrounding the granite of Dartmoor are described by the same geolo- 

 gists as being of a highly schistose character, suggestive of the pro- 

 bability of their being highly metamorphosed slates. 



Summary. 

 The foregoing facts may be briefly summarized as follows. 

 The rocks of the so-called greenstone district of Penzance belong 

 principally to three distinct classes : — 



* The writer some years ago examined this greenstone, and described it 

 (Phil. Mag. vol. xli. p. 97, 1871) as being probably a diorite which had urder- 

 gone extensive alteration. Since that time the quarry has been extensively 

 worked ; and sections recently prepared have been found to enclose small patches 

 of some augitic mineral. They also contain felspar (of which a portion is 

 triclinic), magnetite, green and brown microlites, a nebulous greenish-grey 

 substance, and apatite. It is therefore more probable that this is a highly 

 altered rock belonging to the class found near St. Michael's Mount and in 

 various other localities in the neighbourhood of Penzance, St. Ires, and the 

 Gurnard's Head. 



f Phil. Mag. vol. t. p. 247. London, 1829. 



n2 



