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rock-mass. These " greenstones " admirably illustrate the impossibility 

 of separating rocks into sharply defined groups. We can describe the 

 rocks, but to give each specimen a name which shall connote its 

 characters is impossible. 



The variations in structure are partly original and partly secondary. 

 The most important secondary structural characteristics are those 

 which depend on the development of foliation. Mr. PHILLIPS frequently 

 refers to the gradual passage from a crystalline massive rock, in which 

 the individual constituents are related to each other as in normal igneous 

 products, to a foliated rock which " does not exhibit any of the character- 

 istics peculiar to igneous rocks." He mentions a rock-mass occurring a 

 little west of St. Austell as exhibiting this gradual change in a very 

 .satisfactory manner. 



The variations in composition are also partly original and partly 

 secondary. The most important secondary characters depend upon the 

 development of chlorite or some form of hornblende at the expense of the 

 original augite ; of leucoxene and ultimately granular sphene or rutile at the 

 expense of the titaniferous iron-ore ; and of water-clear secondary felspar 

 (? albite) and other substances at the expense of the original plagioclase. 

 In describing the rocks we shall deal first of all with the lavas and tuffs, 

 then with the more distinctly crystalline rocks and their metamorphic 

 representatives, and lastly with the hornblendic slaty rocks of more or 

 less doubtful origin. 



Lavas and tuffs occur in the neighbourhood of Plymouth and 

 Tavistock in Devon, and of St. Minver (Pentire Point), and other 

 localities in East Cornwall. They are probably also represented in West 

 Cornwall, but the metamorphism in this region has been so intense that 

 it is impossible to speak with absolute certainty on this point. The most 

 typical lavas are locally known as " dunstones." They are usually of a 

 greenish-grey colour and finely amygdaloidal ; the cavities being filled 

 with calcite, chlorite, and quartz ; zeolites are rare. They correspond 

 to the " diabas-mandelstein " of Continental authors. Owing to their 

 extensive alteration by surface and other agencies it is extremely difficult 

 to determine their original characters. 



Microscopic sections usually show minute lath-shaped felspars in 

 a ground-mass, rendered more or less opaque by opacite and various 

 decomposition products. Recognizable augite is comparatively rare. In 

 a rock from Pentire Point, near St. Minver, it occurs in the granular form, 

 characteristic of many basalts. Ill-defined green alteration products 

 (viridite), are, however, frequently present and doubtless represent an 

 original ferro-magnesian constituent. In some instances (e. g., lavas of 

 Landrake and Honicknowle) porphyritic crystals of plagioclase may be 

 recognized as well as the minute lath-shaped felspars of the ground-mass. 

 In a remarkable rock from Egg Buckland, described by Mr. WOKTH, 

 the porphyritic elements consist of globular holocrystalline- aggregates 

 of twinned felspar (glomero-porphyritic structure). Unaltered basic glass 

 is not known as a constituent of these lavas, but the researches of Mr. 



