236 JOURNAL OF THE PLYMOUTH INSTITUTION. 
look under the microscope than in mass. The mosaic shows two 
customary sizes of grains, with plenty of small interstitial patches. 
The felsitic matter might almost be regarded as developed micro- 
porphyritically. . 
A number of elvans occur in the Plym Valley, specimens of 
which may be found among the pebbles in the bed of that river. 
A very even-textured, cherty-looking, pink variety contains little 
nests of schorl, and weathers to a warm pinkish-buff. Apparently 
allied to this is one in which the schorl is dispersed so generally 
and so finely through the felsitic matrix as to give it a finely 
granular appearance. Here and there distinct crystals of felspar 
are developed ; while in a further example both quartz and felspar 
are porphyritic, some of the crystals being of considerable size. 
The base continues distinctly felsitic. There is a rock of much 
the same character near Cornwood, mainly a dull, deep-pink, 
massive glassy felstone, with semi-conchoidal fracture. 
Another Plym Valley elvan is pale-buff, distinctly granular, 
with an occasional fleck of white mica, and a quartz crystal, or a 
cavity from which the crystal has disappeared. A very similar 
rock from Shaugh, in the number of its quartz crystals and 
cavities closely resembles the Roborough elvan, but has occasional 
needles of schorl. A fine-textured, whitish quartz-porphyry— 
both quartz granules and crystals and felspar crystals being 
developed in a close felsitic matrix, slightly speckled by schorl— 
occurs near Trowlesworthy. Not far distant is an elvan with 
black mica and schorl in a granular base. 
A further variety from Shaugh, dirty whitish-brown in colour, is 
almost wholly composed of quartz and felspar distinctly crystallized, 
the felsitic matrix being reduced toa minimum. As other examples 
show, the distance is not very great between this and true granite, 
mica and schorl occasionally making their appearance, and the 
whole rock being essentially crystalline. 
A rock of this class forms the junction at Shaugh, at one point, 
between the true granite and the schist. Essentially a quartz- 
porphyry, it contains a vein which is a fine-grained granite, and 
into which the mica appears to have segregated. 
A variety of the same class, from Cadover, tolerably even- 
textured and semi-granular, contains schorl in little strings, irregu- 
larly dispersed ; and this again has a neighbour that is really a 
fine-grained schorlaceous-granite. 
