97 



rock is soon to consist of sorpcntinous pseudomorphs after olivine, together 

 with a pale brown or claret coloured augite and a very little felspar. Iron 

 ores, pyrites and apatite are also present. 



The rock of Clicker Tor, Menheniot, near Liskeard, Cornwall, presents 

 many analogies with the Inchcolm rock, and must, at any rate, in part be 

 classed with the picrites, as indeed was pointed out in the description of the 

 Inchcolm rock in Cole's " Microscopical Studies." It consists principally of 

 serpentinous pseudomorphs after olivine together with augite. The amount of 

 felspar in the slides already examined is very small. The rock has already 

 been described and figured (see Fig. 2, Plate II.) One interesting feature of 

 this rock is the occurrence of secondary hornblende (tremolite) in the olivine 

 pseudomorphs. 



Many instances of picrite have been described by Professor Jrnn as 

 occurring in the Tertiary volcanic districts of Rum and the Shiant Isles. 



This completes the list of localities in which augite-picrites have been 

 already detected in the British Islands. The rocks stand in the closest 

 relations to the picrites (palooo-picrites) of Nassau and the Fichtelgebirge. 

 They are intimately related to the olivine-dolerites (diabases) into which they 

 pass by the most insensible gradations. 



We have now to consider the group of rocks for which Professor BOXXEY 

 has proposed the name hornblende-picrite. One of the most striking members 

 of this group is the rock from Penarfynydd which was collected many years 

 ago by Professor SKDGWICK. It is a medium grained, black or greenish 

 black rock. Cleavage surfaces of a black mineral showing the characteristic 

 lustre-mottling may be seen on a fractured surface. They are not however 

 so conspicuous as in most of the rocks belonging to this group. Under the 

 microscope olivine is seen to be by far the most abundant mineral ; more than 

 half the rock is composed of it. As a ride it is remarkably fresh ; traversed 

 only by narrow anastomosing strings of serpentine. Next in importance is a 

 rich brown hornblende, which presents the characteristic pleochroism of this 

 mineral. It occurs in large ophitic plates and plays the role of groundmass 

 to the olivine of which the rock is so largely composed (see Fig. o, Plate VII.) 

 Enclosures of magnetite, probably of secondary origin, are common in the 

 hornblende. Sometimes the cleavage cracks are sharply defined by dusty 

 looking magnetite which has separated out along them. Although the 

 prevailing colour of the hornblende is a rich brown, here and there small 

 patches occur which are perfectly colourless. These patches sometimes 

 extinguish at a slightly different angle from the main mass of the mineral. 

 Professor BOXXEY mentions the occurrence of colourless augite in the same 

 rock. 



Next in importance to the olivine and hornblende is a mineral " which 

 occurs in radiated nests, tufted groups of fibres and associated plates, like a 

 mica." This mineral presents somewhat exceptional characters. In sections 

 at right angles to the perfect cleavage, some portions are seen to be green when 

 viewed with rays vibrating parallel to the cleavage cracks, and a rich golden 

 brown when viewed with rays vibrating at right angles to these cracks. 

 These portions polarise in neutral tints, Other portions are a very pale 



