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southern angle of the small beach in Caerleon Cove, about a stone's throw 

 from the Poltesco serpentine works. "It is from 4 to 5 feet wide; the 

 sides for about 6 to 10 inches are very dark and compact, and so platy in 

 structure as to be almost undistinguishable from some specimens of the 

 hornblende schists. This structure is lost rather suddenly ; and the rock 

 assumes the ordinary aspect of an igneous rock, consisting of a finely 

 crystalline mixture of white felspar and dark hornblende with por- 

 phyritic crystals of the former as much as inch long." Under the 

 microscope the ground-mass of the central portions of this dyke consists of 

 pale green or brown hornblende, mostly fibrous (uralitic), felspar either 

 turbid or water-clear and scattered grains of magnetite. The porphyritic 

 felspars are turbid and have lost their individual action on polarized light. 

 The relation of the felspar to the hornblende in the ground-mass leaves no 

 doubt that the original rock was ophitic in texture. It is now an ophitic 

 epidiorite. The rock from the margin of the dyke is principally composed 

 of irregular grains of water-clear felspar, green hornblende and magnetite. 

 A little turbid felspar giving lath-shaped sections is present in some slides 

 and absent in others. The micro-structure of the rock so far as it consists 

 of water-clear felspar, compact hornblende and magnetite is that of a 

 crystalline schist ; not that of a normal igneous rock. Apart from the 

 direct evidence, furnished by the field relations, the occurrence of 

 turbid lath-shaped felspars would give a clue as to the origin of the 

 metamorphosed rock. 



At Coverack and immediately to the north between this village and 

 Manacle Point there occur dykes which have preserved their original 

 characters to a remarkable extent. The least altered of these are ophitic 

 olivine-dolerites. The augite is quite unaltered. It is almost colourless 

 and occurs in fair sized masses, penetrated in all directions by the lath- 

 shaped plagioclase. In one case distinct indications of micro-pegmatitic 

 intergrowths of plagioclase and augite were observed in a slide which 

 showed as a rule the normal ophitic character. The olivine shows no 

 trace of serpentinization. It is, however, grey in colour in consequence of 

 the presence of minute dust-like particles. With a magnifying power of 

 1 ,000 diameters some of the largest of these can be seen to be of a dark 

 reddish brown colour. In some cases the particles are distributed uniformly 

 through the mineral ; in other cases they are aggregated in parallel bands. 

 The olivines are also traversed by irregular cracks along which a segregation 

 of magnetite has taken place. Their general appearance is represented in 

 Fig. 1, Plate XVI. The lath-shaped plagioclase is colourless, brown or 

 turbid. These conditions merely represent different stages of alteration 

 and may be observed in one and the same individual. Turbid plagioclase 

 forms only a very small portion of the entire mass. As a rule the mineral 

 is as fresh as in many Tertiary dolerites. Irregular grains and crystals of 

 magnetite are scattered through the slide and they are very often associated 

 with a brown, uniaxial biotite. The biotite, however, never forms an 

 important feature in the rock. It occurs as irregular scales. It is 

 important to note that the iron ore does not occur in the form of large 



