OF CENTKA.L AND EASTEKN COKNWALL. 491 



posed into a soft yellowish-grey freestone ; sections cut from such spe- 

 cimens are found to consist chiefly of a colourless granular material, 

 in which, although shadowy outlines of felspar are sometimes visible, 

 the crystals of that mineral have generally disappeared. Throughout 

 this are disseminated a few small patches of green hornblende, 

 a little viridite, and some of the greyish dust-like mineral which is 

 sometimes seen to result from the decomposition of augite. 



On an estate called Grove, situate in the parish of St. Stephens 

 (by Saltash), a mile lower down the course of the river JSTottar than 

 Cumble Tor, but on the same side, a quarry was formerly opened 

 upon a band of dolerite of considerable extent and thickness. This 

 rock is rather coarse in its grain, and in hand-specimens appears to 

 consist of a mixture of felspar, in crystals 3 millims. in length, with 

 a dark pyroxenic mineral, also distinctly crystalline and prepon- 

 derating slightly in amount over the felspar. 



In thin sections, examined under the microscope, the plagioclase 

 is seen to be considerably obscured by a flocculent product of altera- 

 tion, while the augite, which in some instances is entirely without 

 change and well crystallized, is in others almost completely converted 

 into uralite. The magnetite is unaltered ; and apatite is compara- 

 tively abundant. Sections prepared from specimens obtained from 

 this locality are exceedingly instructive, affording, as they do, ex- 

 amples of the gradual conversion of an ancient dolerite into a 

 uralitic rock — crystals of augite in all stages of alteration being 

 abundant. 



Near the hamlet of Burraton Combe, immediately north of Tre- 

 maton Castle, a band of dolerite has been laid open by a quarry. 

 The rock here met with, in addition to its normal constituents, 

 contains a little viridite, but is apparently without hornblende ; its 

 ilmenite is much altered ; and it encloses numerous needles of 

 apatite. 



Adjoining the limekiln at Anthony Passage, a mile south of Bur- 

 raton Combe, is a rock consisting of a mixture of plagioclase, augite, 

 hornblende, brown mica, magnetite, and apatite ; some portions of 

 this rock, which is very dark in colour, appear to afford indications 

 of its having been originally, to some extent, vesicular. 



At Wearde, on the eastern bank of the creek extending up the 

 valley in the direction of Ford, and probably upon the same band of 

 rock, a quarry has been extensively worked for road-material. The 

 stone here obtained varies considerably both in texture and in 

 colour, but is generally made up of a nearly equal mixture of dark 

 crystalline minerals with crystals of felspar, the latter being about 

 2*5 millims. in length. Less frequently either the felspar pre- 

 ponderates, giving rise to a rock of lighter colour, or the coloured 

 minerals become more abundant, and a very dark green rock is the 

 result. 



A specimen of the more abundant variety of this rock, first 

 described, was analyzed, in duplicate, and afforded the following 

 results (sp. gr.=2-89):— 



