476 J. A. PHILLIPS ON THE SO-CALLED " GREENSTONES" 



Tregorrick, and Hallane, they were originally augitic rocks which 

 have been subjected to alteration. 



Slaty Agglomerates. — In addition to the doleritic rocks above 

 described, which, under the name of " blue elvans," are largely 

 employed as road-material, there are others in this neighbourhood 

 which, although known by the same name and employed for similar 

 purposes, are manifestly of sedimentary origin. 



A rock of this description which is quarried at Tregian, in the 

 parish of St. Ewe, is of a dark grey colour, distinctly lamellar in 

 structure, and not unfrequently encloses flakes of a dark blue clay- 

 slate. Under the microscope it is seen to be mainly composed of 

 slightly rounded grains of quartz, with fragments of felspar joined 

 by a transparent cement, through which microlitic viridite is dis- 

 seminated ; it also occasionally encloses iron pyrites and minute 

 flakes of silvery-white mica. A very considerable proportion of 

 the felspar, but not quite the whole of it, is plagioclase. 



Similar to the above is a rock worked for road-metal at Dairy, in 

 the same parish. The stone quarried in this locality, however, is 

 darker in colour, contains a larger number of fragments of blue 

 killas, and is more distinctly fissile than that found at Tregian. 

 Sections examined under the microscope are seen to be largely 

 composed of quartz granules, of which the outlines are generally 

 not perceptibly rounded, together with grains of felspar and frag- 

 ments of hornblende, the whole being united by a transparent 

 cement enclosing a little white mica with grains of partially hydrated 

 magnetite. This ground-mass is traversed by numerous feathery 

 microlites. 



A rock somewhat resembling this, although of a dull brown 

 colour, and which might perhaps be more correctly classed as a 

 sandstone, is quarried near the " Bock House " at Pentewan. This 

 is largely composed of quartz granules, consolidated by a cement 

 containing flakes of white mica, and a few patches of viridite, the 

 whole being more or less coloured by hydrated ferric oxide. 



Central Northern District. — The next extensive "greenstone" 

 region lies on the northern coast of the county, reaching from 

 Trevose Head on the west, to beyond Camelford on the east, and 

 extending in a southerly direction a little beyond the parish 

 church of Egloshayle. 



Near the coast-line these rocks occur in nearly parallel bands, 

 of which the direction approaches closely to east and west ; but in 

 the vicinity of the granitic area, of which Brown Willy is the 

 culminating point, they approximately coincide with the lines of 

 its general contour. Ash-beds also occur along the northern coast. 

 The rocks in this district, marked in the Survey-Map as " green- 

 stones," belong to at least three distinct classes — namely, crystalline 

 pyroxenic rocks, vesicular lavas, and foliated ash-beds. 



In the neighbourhood of Wadebridge the igneous rocks belong 

 to the first of these classes, and closely resemble the altered dole- 

 rites of Penzance and St. Austell. In some cases the pyroxenic 

 constituent remains partially unaltered, while in others it has not 



