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Some of the rocks have a massive habit and are closely allied to gabbro in 

 composition and structure ; most of them are gneissic. They have affinities 

 with the ariorthosite (Norian) rocks of Canada, and with the trap-granulites of 

 Saxony. Many, if not all, of these were probably eruptive, but as this point 

 is at present undetermined we shall describe them along with other rocks of 

 doubtful origin. 



The Lizard district. Turning now to England we note that the most 

 important exposures of gabbro are undoubtedly to be found in the Li/ard 

 peninsula of Cornwall. They occur as large intrusive masses, and also as 

 veins and dykes in the serpentine. The largest mass is that of Crousa 

 Down. This is represented on the geological map as occupying an area of 

 six or seven square miles. It forms the coast line between Coverack and 

 Manacle Point, and is well exposed in low cliffs along the shore. Veins and 

 dykes of a fine-grained " greenstone " (epidiorite or pseudo-diorite) occur 

 in the gabbro, and these increase in number towards the north, until in the 

 neighbourhood of Porthoustock the greenstone is the dominant rock. 



South of Coverack the greater part of the district is occupied by serpentine, 

 in which veins and dykes of gabbro frequently occur. The mass which ranks 

 next in importance to that of Crousa Down is exposed on the sea-coast west 

 oi Lankidden Cove. It forms the headland of Karakclews, and may be 

 followed along the coast for a distance of about a quarter of a mile. A mass 

 of similar gabbro occurs inland at Gwinter. The two portions are doubtless 

 continuous, and, if so, they form a dyke-like mass running inland in a N.W. 

 direction. The Karakclews gabbro is bounded on both sides by serpentine 

 into which it sends veins and dykes. Another considerable exposure occurs 

 on the shore north of Pen Voose, near Landewednack. 



The principal constituents of the Lizard gabbro are plagioclase, augite or 

 diallage, hornblende, olivine, and the dense white opaque substance known as 

 saussurite. Iron oxides, except such as are undoubtedly of secondary origin, 

 are rare or altogether absent. 



The least altered rocks bear the closest possible resemblance to the 

 Tertiary gabbros of the west of Scotland. In these rocks the plagioclase 

 occurs in the form of grains of tolerably uniform dimensions in the different 

 directions. The grains show broad lamellar twinning, and not seldom two 

 sets of lamellae are seen crossing each other at right angles. They are 

 perfectly fresh. In the more or less altered gabbros we see various stages of 

 the replacement of the felspar by white or cream-coloured saussurite. 



The felspars of certain gabbros in which the development of saussurite 

 has not taken place at all, or has only occurred to a very limited extent, often 

 give evidence of having been affected by profound mechanical disturbances. 

 The twin lamellae are often bent. The extinction under crossed nicols is 

 wanting in sharpness and definiteness. Cracks traverse the crystal and a 

 felspar-mosaic has often been developed along the cracks. The final stage of 

 this kind of metamorphism is represented by the replacement of the original 

 crystal by a water -clear felspar-mosaic. These changes in the felspar are of 

 course accompanied by changes in the other minerals, and by a modification of 

 the original structure of the rock. Specimens of a felspar-diallage gabbro 

 (eucrite) from Gwinter show the above features in the greatest perfection. 



