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Head and St. Ives. Not unfrequently they pass by imperceptible gradations 

 into more or less foliated rocks Near the pier at St. Ives there occurs a 

 very fine ophitic proterobase with traces of the effects of dynamic-meta- 

 morphism. The augite is pale in colour and passes into hornblende at the 

 edges. The lath-shaped felspars have been more affected than the augites. 

 They polarize as aggregates rather than as individuals. The slide is 

 traversed by bands along which shearing has taken place so that the 

 characteristic structure of the igneous rock has been partially destroyed. 

 At Carack Olu, also near St. Ives, the metamorphism has been carried a 

 stage further. A rock from this locality may be described as a foliated 

 epidiorite. Traces, however, of the original ophitic structure remain. The 

 foliation is not even and regular as in a typical schist, but of the flaser 

 type. Ophitic epidiorites occur at many points in Cornwall. They have 

 been observed at Gurnard's Head, Penlee Point, Marazion, Newham near 

 Helston, and many other localities. There can be no doubt, therefore, 

 that many of the highly metamorphosed rocks of West Cornwall were 

 originally ophitic dolerites, essentially similar in structure and composition 

 to those which occur in a comparatively unmetamorphosed condition at 

 many points in East Cornwall and Devon. Nevertheless some of the rocks 

 appear to have contained idiomorphic augite. 



In addition to the more or less foliated rocks which can be definitely 

 recognized as metamorphosed dolerites, Mr. PHILLIPS describes a number 

 of slaty greenstones about the origin of which it is not possible at present 

 to speak positively. Some of these resemble the metamorphosed dolerites 

 in chemical composition ; others differ from them in a remarkable manner. 

 The former consist essentially of interlacing aggregates of actinolitic horn- 

 blende sometimes associated with a certain amount of chlorite, a colourless 

 water-clear substance having the refraction and double refraction of quartz 

 and felspar (probably in most cases secondary felspar) and more or less mag- 

 netite or ilmenite in a granular form. Garnet, tourmaline, and axinite 

 are sometimes present as accessory constituents where the rocks occur 

 near granite masses or large elvan dykes. The water-clear substance forms 

 the matrix in which the hornblende is embedded. Examples of this class 

 occur near Botallack, in the district between St. Erth and Camborne, 

 and doubtless in many other localities. In chemical composition (see 

 below) and also in specific gravity the rocks resemble the proterobases 

 and epidiorites. They may be metamorphosed dolerites, basalts or 

 basalt-tuff's. 



The slaty greenstones which differ from the proterobases and 

 epidiorites in composition have been recognized only in the Penzance 

 district. They may be described as highly basic hornblende rocks. They 

 are dark green in colour and frequently show a spotted aspect. Under 

 the microscope the spots may be resolved into aggregates of interlacing 

 actinolite, sometimes associated with a water-clear colourless mineral. The 

 general mass of the rock is composed of hornblende, chlorite, granular 

 opacite or leucoxene, epidote and a very small quantity of the colourless 

 water-clear substance. The relative proportions of the above constituents 



