rock is argillo-ferruginous,[1] and by itself does not effervesce with acids, as it is easy to prove by making use of pieces of the pure sandstone for that purpose; but it produces so brisk an effervescence from the intimate mixture of calcareous particles, that it might be very easily mistaken for limestone. The substances which enter into the composition of this conglomerate are numerous; and it may first be remarked, that these pieces are of very different sizes and forms, sometimes rolled and rounded, sometimes pointed with sharp angles, from very minute grains to the size of several inches in diameter. There are found in it rhomboidal crystals of calcareous spar, and crystals of felspar, most frequently of an opaque white, and decomposed; pieces of flint; grauwacke; yellowish limestone; rolled masses of a species of porphyry somewhat resembling the antique, the base of which is of a reddish brown colour, not effervescing with acids, and containing numerous, small and well-defined crystals of felspar imbedded in it; pieces of a rock which is itself compounded, having the appearance of a porphyry, the base earthy, and including small grains of quartz, crystals of felspar, and pieces of bluish carbonate of lime; and a whitish tender steatite, in small angular fragments.
Poucham quarry is situated about two miles N. N. W. of Exeter. It is of much less extent than that in the parish of Heavitree. I was not able to determine either the direction or inclination of the
- ↑ May not the presence of iron in this conglomerate be owing to the disintegration of the granite of the low chain of Cornwall, theo mica of which contains so great a proportion of it in the state of oxide?
The Count de Bournon appears even disposed to believe, that the primitive rocks have been the only source from which the iron has come, that is spread over the whole surface of the earth. Traité de Min. vol. I. p. 62.
