THE ROCKS OF PLYMOUTH. 235 
crystalline, admixture of orthoclase and quartz, in which crystalline 
granules of plagioclastic felspars not unfrequently occur.” This 
definition covers a wide range of characters, and when the 
porphyritic constituents are taken into account also, we find the 
term elvan to have a very broad and vague application. All dykes 
of granitoid matter, not distinctly granitic, pass indeed under this 
name. By the aid of polarized light, however, the microscope 
reveals a characteristic structure of the true elvan—a brightly- 
coloured minute mosaic, due to the intimate mixture of the crystals 
and granules of the quartz and felspar in the matrix. 
Apart from those directly associated with the granite, we have 
five elvan courses within a few miles to the north of Plymouth. 
The nearest and best adapted for study stretches from Cann 
Quarry, by Colwell, to near Knackersknowle. At Colwell it is 
worked for road metal, and about thirty feet wide, striking five 
degrees south of west, with a dip of sixty-five degrees south. 
Another elvan dyke, taking the same general direction—as all the 
elvans of this district do—runs from Bickleigh Vale towards 
Fancy; and there is yet another close to Jump. But the finest 
example in the neighbourhood is that which traverses Roborough 
Down, stretching for three miles from the Tavy at Lophill to 
within half a mile of the Meavy. This elvan is quite historic, 
and was largely used in building operations in this district in the 
Middle Ages, being a free-working material of great durability. 
Roborough Rock is the projecting portion of another elvan, which 
differs mainly from its companions in its more quartzose aspect. 
Microscopically these rocks all present the mosaic characteristic 
of felsitic matter under polarized light. The base of the Roborough 
elvan is a very fine granular mosaic, scattered through which are a 
number of pyramidal limpid quartz crystals, and hollows whence 
similar crystals have disappeared, occasionally lined with a yellow- 
greenish crust. The Colwell elvan is very similar, but quartz 
crystals seldom occur. There are a few flecks of mica, and some 
dendritic markings. There are casual traces, apparently, of schorl. 
In Cann Quarry this elvan is rather more irregular in character. 
There is more mica, more quartz, and felspar is also porphyritically 
developed ; on some of the joint faces there are brilliant prisms 
of rock-crystal, with cubes and other forms of pyrites. 
The Jump elvan is microscopically of finer grain than either of 
the others. Roborough Rock has a more characteristic elvany 
