ing chapter, that granite, by becoming finer grained, frequently passes to the state of porphyry. The eurite of the French geologists, and the weiss-stein or white stone of Werner, is a granite in which the felspar is the principal constituent part, and is either finely granular or nearly compact. To this variety English geologists give the name of compact felspar the white elvan of the Cornish miners is a porphyritic eurite.

Geologists have described four formations of porphyry, but it is generally agreed that there is much uncertainty with respect to the situation of these formations. The porphyry which occurs imbedded in granite, or which appears to be formed by a change of structure in that rock, may properly be classed with primary rocks: it is not considered to be an extensive formation; the white elvan of Cornwall, and probably the porphyry associated with mica-slate in Argyleshire, belong to this formation. Porphyry also occurs in enormous masses covering both primary and transition rocks unconformably; but this porphyry belongs more properly to transition rocks, and will be described with them in the following chapters. Porphyry is sometimes a volcanic rock, and appears to form the connecting gradation between granitic rocks, and those of igneous origin. Before taking leave of the rocks classed as Prima

ry,

it may be proper to notice that some of the rocks associated with granite, gneiss, and mica-slate, occur also in the transition class, and even in the lower secondary strata. The same causes by which they were formed among primary rocks, have also operated at a later period: indeed one of the well known

rocks, limestone, has been deposited or formed in all the different classes of rocks except the volcanic, and must therefore receive its name from the class with which it is associated; as primary limestone, transition limestone, &c. In some instances the mineral characters, or.the fossils, serve to distinguish rocks of the same kind, that occur in the different classes or formations: thus the rocks associated with primary rocks are generally harder and more crystalline than the same species of rock which occurs in the secondary class; but this is not invariably the

case.

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CHAPTER VII.

ON TRANSITION OR INTERMEDIATE ROCKS. Character and Classification of Transition Rocks.-Slate the Clayslate of Werner.-Roof Slate, its Cleavage not the Effect of Stratification, but of Crystallization.-Talcous Slate.-Whetstone Slate. Flinty Slate. Greywacke and Greywacke Slate, its Passage into Red Sandstone and Gritstone.-Errors of English and Foreign Geologists respecting the Old Red Sandstone and Mountain Limestone.-Old Red Sandstone.Transition Limestone.-Transition Limestone of Devonshire, and Dudley in Staffordshire.-Upper Transition or Mountain Limestone.-Its Connection with Coal Strata.-Observations on the Fossils in Transition Rocks.

TRANSITION or intermediate rocks cover rocks of the primary class, and are distinguished as the lowest rocks in which the fossil remains of animals or vegetables are found; they may be regarded as the most ancient records of our globe, imprinted with the natural history of its earliest inhabitants.

Transition rocks are the principal repositories of metallic ores, which occur (both in veins and beds) more abundantly in many of the rocks of this class, than in primary rocks. Metallic veins very rarely occur in the secondary strata.

Geologists have often been perplexed in their attempts to draw a well marked line of distinction between primary and transition rocks: the difficulty has arisen chiefly from their arranging slate with the primary class; and hence the disciples of Werner

have been obliged to introduce the theoretical terms of newer and older primary slate, and newer and older transition slate, &c. If the occurrence of organic remains in rocks be the characteristic distinction between the primary and transition class, slate must certainly be classed with the latter; for it is among slate rocks that the fossilized remains of animals and vegetables first appear, in every country that has yet been examined. One of the disciples of Werner, M. D'Aubuisson, admits that there is no where any extensive formation of primary slate. M. Bonnard, another disciple of the same school, in his Apperçu Geognostique des Terrains, after enumerating various primary slate rocks, candidly acknowledges that it is doubtful whether primary slate can any where be found. It is true, that micaslate passes by almost imperceptible gradations into common slate; but here, as in other instances, we only find that Nature is not limited by the artificial arrangements of the geologist: yet so long as it may be proper to class rocks containing organic remains, with transition rocks, we must place slate among them. Nor can this be invalidated by the fact, that in some slate rocks no vestiges of animal or vegetable remains occur; for among the secondary strata, abounding in such remains, we often meet with alternating beds, in which they are never found; but we do not on that account class them with primary rocks. In arranging transition rocks, I most decidedly place the English mountain limestones among them, as I have done in the former editions of this work. I know no circumstance in Geology that evinces more strongly the tenacity

with which errors are cherished, when they have been some time entertained, than the determination of English geologists to separate mountain limestone from transition limestone,-in opposition to analogy, and to the universal opinion of geologists on the continent. This separation, as a mere matter of classification, would be in itself of little importance; but it has tended more than any other circumstance to perplex both foreign and English geologists, in their attempts to assimilate the rock formations of England, with those on the continent of Europe.

When a general attention was first excited in this country to the study of Geology, access to the continent was extremely difficult, and we were left to explore as well as we could the geology of our own island, enlightened only by the dark-lantern of German Geognosy. Many characters were given of transition rocks, and floetz or parallel rocks, founded on local observations in Germany, which did not apply to the rocks in other countries: it was found that the characters of our metalliferous limestone did not agree very well with either, and therefore English geologists have retained the name of mountain limestone, and the appellation of transition limestone was restricted to a lower bed, small in extent, and comparatively unimportant. When I first visited the continent, and examined the cabinets of some eminent geologists, I was particularly struck with finding the analogues of our principal beds of mountain limestone exhibited as types of true transition limestone. On my return to Paris the following year, I took specimens of our moun