CHAPTER VII.

ON INTERMEDIATE OR TRANSITION ROCKS.

Characters and Classification of Transition Rocks.-Slate or Clay-Slate.-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 distinction between the primary and transition class, slate must certainly be classed with the latter; for it is among the 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 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 mica-slate 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

ERRORS RESPECTING THE CLASSIFICATION OF TRANSITION ROCKS. 83

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 fleetz 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 Shropshire 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 Calcaires 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 :

TRANSITION CLASS (conformable).

1. Slate, including flinty slate and other varieties.

2. Greywacke and greywacke-slate, passing into old red sand

stone.

3. Transition limestone. Mountain limestone.

Rocks covering Transition Rocks (unconformably).

4. Porphyry, passing into trap or green-stone.

5. Clink-stone, passing into basalt.

6. Basalt.

Strata covering Transition Rocks (conformably).

7. The coal measures.*

Slate of which roof-slate is a well known variety-is called by the Germans Thon-scheifer or clay-slate; by ancient English geologists, argillaceous schistus; by the modern French, Phyllade. The term slate is perhaps the most proper that can be used to designate this rock as the best variety of it, Roof-slate, is well known. Clayslate is a name given from an erroneous opinion respecting its constituent parts; and the term is liable to create much confusion, as the softer kind of slate in the coal strata is called slate-clay. I shall, therefore, throughout the present volume substitute the term slate for clay-slate, and for slate-clay the more intelligible English term shale.

Slate rocks abound in most alpine districts, resting either on granite, gneiss, or mica slate. That slate which lies nearest the primary rocks has a more shining lustre than the other, and partakes more of the crystalline quality of mica-slate. As this rock recedes from the primary, its texture is generally more earthy. Its colours are various shades of gray, inclining to blue, green, purple, and red. Some kinds of slate split into thin laminæ, which are well known as forming roof-slates. Slate rocks are commonly divided into beds of various degrees of thickness, which generally are much elevated, and from the natural divisions of the rock, they often form peaked

and serrated mountains.

Slate has been described by former geologists as distinctly stratified, because it splits easily into thin laminæ, and the direction of the laminæ is asserted to be in the direction of the beds; but, in opposition to the authority of many eminent geologists, I maintain that slate, unless it be of a soft or coarse kind, approaching to shale or greywacke, invariably, splits in a transverse direction to that of the beds, making with that direction an angle of about sixty degrees;it has frequently two distinct cleavages.

Few persons, perhaps, have examined more slate rocks, or consulted more workers in slate quarries than I have; and the fact respecting its cleavage is invariably what is here stated, except in very coarse greywacke-slate, and soft slate or shale.

The regular coal strata or coal measures, where they occur in England, separate the transition from the secondary rocks. If they are classed with either, it should be with the former.

Slate rocks vary much in quality in the same mountain; those which contain a great quantity of siliceous earth pass into flinty slate. When magnesia enters largely into the composition of slate rocks, they are distinguished by their green colour, and pass into chlorite or talcy slate, a rock before mentioned as occurring also in primary mountains. Whetstone-slate, or hone, is a variety of talcy slate, containing particles of quartz: when these particles are extremely minute, and the slate has a uniform consistence and requisite degrees of hardness, it forms hones of the best quality. Carbonaceous matter is first discovered in slate rocks, and increases in quantity as they approach the secondary strata. Drawing-slate is said to contain 11 per cent. of carbon; where the carbon is very abundant, the slate has a dark colour, and is generally soft. Impressions of vegetables are found in some slate rocks that were formerly regarded as primary; the slate rocks in the vicinity of Mont Blanc, and Mont Cenis, contain impressions of ferns. Slate contains occasionally impressions of fuci, or sea weed.

That fine variety of slate which is used for roof-slate, seldom forms entire mountains, but is generally imbedded in slate rocks of a coarser kind: the beds of roof-slate are sometimes of considerable thickness, and generally rise at an elevated angle. If geologists had not been induced, by an attachment to theory, pertinaciously to adhere to opinions once received, they could not have failed to recognise the effect of crystallization in the cleavage of slate, as evidently as in the laminar divisions of felspar.

Those varieties of roof-slate are preferred for the covering of buildings, that are the least absorbent of water, and have the smoothest surface, and split into the thinnest plates; they are, however, frequently made too thin to be durable, and too light to resist the force of the wind, during storms.

Quarries of slate are worked extensively in Westmoreland, Yorkshire, Leicestershire, North Wales, Cornwall, and Devonshire. The foreign localities of slate are so numerous, that it would be superfluous to name them.

Mountains of slate are seldom so precipitous as those of granite, but have often a sharp serrated outline. They are covered with verdure on their declivities, as they contain less silex, and a more equal admixture of the earths favourable to vegetation.

Flinty slate, as before observed, differs from common slate by containing a greater quantity of siliceous earth; and, as its name implies, it partakes of the nature of flint. Slate and flinty slate not only pass into each other, but frequently alternate. When the latter ceases to have the slaty structure, it becomes hornstone, or what the French denominate petrosilex. If it contains crystals of felspar, it becomes hornstone porphyry: all these varieties may be observed alternating with each other in the same rocks in Charnwood Forest, and in North Wales and in Cumberland.

Slate is regarded as one of the most metalliferous rocks nearly all the principal metallic ores have been found in slate, either in veins or beds; but it is remarkable that flinty slate seldom contains any repositories of metallic matter. Lead and copper are the principal metals found in the slate rocks of England and Wales: they are not so rich in lead as the mountain limestone, but the lead ore in slate rocks contains a larger portion of silver. The killas of Cornwall, so remarkably metalliferous, is a variety of slate.

Greywacke and Greywacke-Slate; German Grauwacké.-This dissonant term, which we have borrowed from the German, the French geologists have exchanged for a name not more harmonious, though more expressive, Traumate, from the Greek Thrausma, a fragment.

Greywacke, in its most common form, may be described as a coarse slate containing particles or fragments of other rocks or minerals, varying in size from two or more inches to the smallest grain that can be perceived by the eye. When the imbedded particles become extremely minute, greywacke passes into common clay-slate. When the particles and fragments are numerous, and the slate in which they are cemented can scarcely be perceived, greywacke becomes coarse sandstone or gritstone. When the fragments are larger and angular, greywacke might be described as a breccia with a paste of slate. When the fragments are rounded, it might not improperly be called an ancient conglomerate. When rocks of greywacke have a slaty structure, they form greywacke-slate.

Greywacke has by some of the French geologists been described as a transition sandstone, with a cement either of siliceous earth, or of slate. This definition agrees with the gritstones associated with the upper transition or mountain limestone. Where the paste is hard and siliceous, as I have observed in the greywacke of Savoy, that separates the primary from the secondary rocks, many of the siliceous particles may have been original concretions formed at the same time as the paste; and where these concretions are all composed of quartz, we may infer that such has been their mode of formation. In other instances, the fragments are evidently the débris of more ancient rocks, that have been broken down by some great catastrophe, and mixed with more recent beds at the period when they were forming. This mode of formation implies, that a considerable period elapsed between the formation of the primary and secondary rocks. The fragments are always those of lower rocks, and never of the upper strata. In some situations, immense beds of loose conglomerate, composed of large fragments and boulders of the lower rocks, separate the slate rocks from the calcareous formations: such conglomerates may be regarded as occupying the geological place of greywacke, and belonging to the greywacke formation.