422 S. ALLPORT ON THE METAMORPHIC ROCKS 



this mineral is seen to be much decomposed, although the original 

 striation is here and there quite distinct ; the augite is hut slightly 

 altered, and many of the crystals are well developed. 



The Botter Rock, forming part of the same mass, but nearer the 

 granite, is rather coarsely crystalline, dark greyish-black in colour, 

 and without felspar porphyritically enclosed. A thin slice exhibits 

 quite similar characters to the last, except that the augite is rather 

 more altered, and there is much more flocculent green matter dis- 

 seminated through the mass : there are also numerous rather large 

 crystals of apatite, which appear either as long prisms or perfect 

 hexagons. 



St. Austell. — At a place called the " Sanctuaries " near this town, 

 and just outside the band of altered slate surrounding the granite, 

 the largest mass of "greenstone" is also an altered dolerite, some 

 parts of which are highly instructive. Part of the augite is un- 

 altered; some of the crystals have assumed a brown granular 

 appearance, while others have been converted into the hornblendic 

 substance previously described. Apatite is abundant ; felspar cry- 

 stals are distinct, though much altered ; and there is a considerable 

 quantity of actinolite, filling cavities and the angular spaces be- 

 tween the larger crystals. 



Helston. — The "greenstone" to the north of Helston occupies 

 precisely the same relative position to the granite as the rock just 

 mentioned. The specimens examined, however, are far more highly 

 metamorphosed, and might almost be described as hornblende-schist; 

 the original character of the rock is nevertheless apparent from the 

 remains of felspar and other crystalline forms. 



Having described several examples of dolerites occurring among 

 the altered slates close to the granite, it will now be well to examine 

 some of those found in the ordinary " killas," or clay-slate, at a dis- 

 tance from the altered band. 



Clicker Tor. — This hill is close to the Menheniot Railway Sta- 

 tion, about three miles south-east of Liskeard ; it forms the mass 

 coloured serpentine on the map of the Geological Survey. It is an 

 elongated ridge having a nearly east and west direction, and rising 

 rather abruptly from the surrounding undulating country ; its gene- 

 ral aspect is quite that of an intrusive mass ; and an examination of 

 its relations to the adjacent deposits shows clearly that such is its 

 true character. At the east end of the hill all the rocks are well 

 exposed in the railway-cutting ; and in the section exhibited close to 

 the station Devonian slates are seen on both sides of the central 

 mass. On the south side the Hue of junction is nearly vertical, 

 and the ends of the highly inclined and contorted slates lie directly 

 against the trap. Under the microscope, a thin slice of the in- 

 trusive rock exhibits a variegated mass of pale green serpentine 

 and a nearly colourless substance intimately blended together; 

 imbedded in this matrix there are numerous pseudomorphs after 

 olivine, and irregular plates of unaltered augite together with minute 

 grains of magnetite scattered here and there through the mass. 

 The pseudomorphs after olivine are of two kinds, consisting either 



