Yol. 59.] THE TINTAGEL AND DAV1DST0W DISTRICT. 417 



rods of micaceous ilmenite, and a grain of apatite are the accessory 

 minerals. 



In this place may be mentioned a thin bed of limestone which 

 occurs on the eastern side of the Eocky Valley, and is traceable 

 elsewhere at the same horizon as a greyish-white compact rock, 

 with a very slabby fracture produced by the presence of mica- 

 scales. This rock and its varieties effervesce energetically with 

 hydrochloric acid in the cold. Thin sections show the rocks to 

 consist of calcite, with a few quartz-grains, flakes of tourmaline, and 

 sericitic mica as accessories. Dr. G. J. Hinde, E.R.S., who has 

 kindly looked at one slide, tells me that it contains echinoderm- 

 fragments. 



This limestone-bed occurs at the base of, and is inseparable from, 

 the Volcanic Series, so that in some instances the members of the 

 latter may have been extruded as lavas or deposited as tuffs under 

 conditions which were suitable for the formation of a limestone. 



In the rocks now to be described carbonates are conspicuous, and 

 appear to be in part the infilling of vesicles. 



Scattered apparently fortuitously among the other members of 

 the Volcanic Series, these rocks are distinguished by the presence 

 of calcite and a yellowish-green mica, the former often concen- 

 trated in bands. On the coast, the upper part of the cliff on the 

 western side of Bossiney Haven and the lower part of Smith's Cliff 

 provide excellent sections ; and, inland, scattered outcrops appear at 

 Tregrylls, Tremail, Treglasta, Trevenn, and elsewhere. 



The rock at Bossiney Haven contains irregular lenticles of calcite, 

 about *5 x '15 of an inch in size, which pass abruptly into the sur- 

 rounding rock (fig. 3, p. 418). A greenish mica and some crystals of 

 epidote are conspicuous on foliation-surfaces, the individual flakes 

 of the former being just discernible to the naked eye, while the 

 crystals of the latter attain a tenth of an inch in length. 



Carbonates are conspicuous in a thin section, the grains being 

 arranged in irregular ellipses, up to -15 inch in length, bordered and 

 penetrated by well-formed flakes of yellowish-green mica. The 

 latter mineral also commonly forms lenticular patches, in which 

 many crystals have grown transversely to the foliation. Locally 

 they are studded with sphene. Doubtless the colourless crypto- 

 crystalline groundmass in which these lenticles of calcite lie repre- 

 sents ' in a general way the felspathic constituent of the original 

 rock,' as in the most metamorphosed of the basic lavas, described 

 by Messrs. Harker & Marr l from the neighbourhood of the Shap 

 Granite. This groundmass is spotted with crystals of epidote, flakes 

 and grains of opacite, magnetite, and ilmenite, granules of sphene and 

 rutile, and innumerable minute specks of mica. Earlier in forma- 

 tion than the large mica-flakes above mentioned, but associated 

 with them, are the faintly-pleochroic crystals of epidote, 2 most 



1 Quart. Journ. O-eol. Soc. vol. xlix (1893) p. 362. 



2 For a description of the epidote in this rock, see W. M. Hutchings, Geol. 

 Mag. 1889, p. 105. 



