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It is difficult to account for these differences in the same rock, but though they certainly do exist, I do not think they are sufficient to constitute distinct species. I conceive that in the nomenclature of rocks especially, we ought strictly to adhere to the principle laid down by Saussure—“ To determine genera and species from those individuals in which the characters are most distinct, and to qualify by the name of transitions, those which are doubtful or indistinct.”

From Lynher Creek, directing our course northward towards Callington, by Saltash and St. Mellion, the grauwacke continues uninterrupted, but as the country rises, and we approach Kitt—hill, situated about a mile N.E. of Callington, the quantity of quartz met with on the road in blocks and pebbles becomes very remarkable: this indicates our being near the termination of the grauwacke, and we accordingly rind it cropping out to-day on the side of Kitt-hill, at about two-thirds of the ascent. Kitt-hill is the nearest place to the sea from the mouth of the Hamoaze, where granite is found in situ. This little hill, though insulated, must be considered as a dependence of the mountains of Dartmoor: it is situated on the southern skirt of the granitic mountain-plain of the low range of Cornwall. The sides of Kitt-hill are gently inclined; that on the east is the most abrupt. The north and south sides are the most extended, and may be considered as the water-sheds. The upper part of the hill is a true granite, composed of crystals of white felspar, quartz and mica. Mr. Necker found in the neighbourhood of Kitt-hill an adventitious mass of tourmaline of a cylindrical form, and of an brownish-green colour, which the Comte de Bournon, to whom I shewed it, considers a new variety of form. From Callington to Plymouth, by Beer Alston, Beer Ferris, and Tamerton Folliet, as far