applied had a purple colour. In the modern acceptation of the term, any rock which is compact or finely granular, and contains distinct imbedded crystals, is called Porphyry, whatever be its colour. The base or paste of most porphyritic rocks is felspar; and the imbedded crystals are also felspar, though there may be also small grains or crystals of quartz or other minerals. It has been stated, in the preceding chapter, that granite, by becoming finer grained, frequently passes to the state of porphyry. The eurite of the French geologists, and the weissstein 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 regularly imbedded in granite, or which appears to be formed by a mere 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, sometimes intersecting and sometimes covering primary mountains. The granite of Ben Nevis in Scotland is intersected by veins of porphyry; and at the head of Glen Ptarmagan, a cliff of porphyry 1500 feet high, shaped like an oblique truncated pyramid, passes through granite. Porphyry; imbedded in transition rocks, or associated with trap or volcanic rocks, must generally be regarded as cotemporaneous with the formations in which they occur. Porphyry is in some instances an undoubted

* Phil. Mag.

volcanic formation, and presents a connecting gradation between granitic primary rocks, and those of a more recent igneous origin. Wherever porphyry occurs unconformably, covering other rocks, it is evidently more recent than the rocks on which it rests, and must be classed with basaltic or trap-rocks; this porphyry will be described with them in a subsequent chapter.

Before taking leave of the rocks classed as Primaгу, 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 seeondary class; but this is not invariably the case.

CHAP. VII.

ON INTERMEDIATE OR TRANSITION ROCKS.

Characters and Classification of Transition Rocks. Slate or ClaySlate. Peculiarities of Structure. — Varieties of Slate. Flinty Slate. Greywacke and Greywacke-Slate; its Passage into Red Sandstone and Gritstone. Errors of Geologists respecting the old Red Sandstone. - Lower Transition-Limestone: remarkable Position of its Beds. - Upper Transition or Mountain Limestone. Magnesian Limestone, in Mountain Limestone. Peculiarities in the Stratification of Clouds Hill. -Errors respecting the Mountain Limestone of Derbyshire.Remarkable Structure of Crich Cliff. — Quartz Rock. - Jasper Greenstone. Coal Strata in England separate the Upper Transition Rocks from the Secondary. - Observations on the Transition Rocks of distant Countries.- Errors of Geologists respecting them.

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 dis

tinction between the primary and transition class, slate must certainly be classed with the latter; for it is among the slate rocks that the fossilised 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 Géognostique 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, or 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 mountain limestone from Derbyshire, Westmoreland, Somersetshire, and Wales; and also of the lower limestones from Shrop shire and Devonshire; and presented them to MM. Brongniart and Brochant. The whole of the specimens they recognised as transition limestones, and selected the encrinal and dark madrepore mountain limestones, as the true types par excellence des Cal caires de Transition.

The following arrangement of transition rocks comprises the lowest rocks in which organic remains occur, and those which are metalliferous, or are associated with metalliferous rocks: