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ONLY one case of interposed pyrogenous rock occurs in the limestone tract of England to the north of Derbyshire, but this is one of the most extensive examples known in geology, and has given rise to several descriptions. Professor Sedgwick and Mr. Hutton, in particular, have examined this rock with much attention and published different views concerning it to ste, nd

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The toadstone of Derbyshire, whether it be a single or triple, mass, may be considered as one great eruption of melted rock interpolated in the limestone series; the Whin sill,' as it is called, of Yorkshire, Durham, and Northumberland, is another. They do not correspond in position among the strata; the toadstone lies among lower beds than the Whin sill. They agree in some respects; both being to a certain extent stratiform, irregular in thickness, variously traversed by faults and veins.

The Whin sill is a great mass of greenstone and basalt, extending from near Brough in Westmoreland to the northern parts of Northumberland, occupying a position more or less definite immediately below or in the lower parts of the Yoredale series, never passing downwards to the Melmerby limestone, nor upwards to the main or twelve fathom limestone. In thickness it varies extremely, and within a small compass; there is generally but one stratiform mass; sometimes two, perhaps three, occur, (as near Bavington, Northumberland.) It has never been seen to pass into any of the numerous fissures of the limestone, series, nor to send veins into their substance, but it occasionally includes

altered portions of stratified rock, and appears geographically related to certain great basaltic dykes. It is traversed, like all the beds of the limestone series, by mineral veins, sparry fissures, and common faults. The rocks above and below it are (locally) metamorphic. It is frequently columnar, and impresses a like structure on some of the beds in contact with it.

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The geological place of the Whin sill is better known in Teesdale, round Cross fell, and along the western edge of the Penine chain from Brough northwards, than elsewhere. From my own inspection, and the statements of miners, I can not doubt that the place of the Whin sill, in Teesdale and Tynedale, and along the breast of the Penine chain, is constantly below, but not far below the Tyne bottom limestone, (which forms the upper part of the great scar limestone of Ingleborough), and above the thick limestone of Melmerby scar. More than this can not be said, because of the variable nature of this part of the series of strata, the augmentation of its total thickness to the north, and the interposition of new limestones, gritstones, and shales, from Murton northwards. It is to this interpolation of new terms that the apparent discrepance of its position at Murton and Rundle beck is owing. This is easily seen by examining a sufficient number of points. It is distinctly seen in this relation, in Hilton beck, at High-cup-nick, in Knock Ore gill, Crowdundle beck, Troutsdale, Maizebeck, Teesdale, and Tynedale. It is from its stratiform character and constant position that the basaltic mass of Teesdale and Tynedale has received its name of 'Whin sill,' in contradistinction to Whin dykes ;' besides being thus definite in its leading relations to the limestones indicated, its surfaces are generally conformed to their planes of stratification. Of this the Holwick scars, the bed of the Tees near Winch bridge, the High force, and the whole western escarpment, appear to me to furnish decisive evidence. But yet there is some variation in this respect, especially in the cliffs which border the Tees below Caldron snout. In this locality, on the left bank of the Tees, the Whin sill appears in Widdybank scars, and, as Professor Sedgwick describes it, unconformably sweeps over the edges of some of the

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stratified rocks. In one locality the trap is based on a singular fragmentary rock of variable composition; in some points it is an argil laceous breccia, reminding one of the red argillaceous rock of Derwentwater; in other parts crystalline limestone, variously coloured and holding different crystals, enters largely into the mixture; some portions have a porphyritic aspect; many look like a volcanic breccia; a few seeming beds occur of a quartzose compound, almost exactly like grauwake, and closely resembling some local beds which cover the slate deposit in Ribblesdale. The variation of the thickness of the basalt is remarkable and sudden in Hilton beck four fathoms, in Knock Ore gill ten fathoms, in Tynedale twenty fathoms, at Caldron snout in Teesdale thirty or forty fathoms. It is a compound of white felspar and black pyroxene, the latter generally predominating; generally fine grained, but where its thickness is considerable showing also coarse granular parts, like some greenstones of Scotland. Contemporaneous veins of greenstone with large curved crystals, (hypersthene?) pass through the general mass near High force and Caldron snout; strings of spar, quartz, and veins of lead ore (Hilton, Dufton, Trout beck, Tynedale, and Bavington,) occur. The Whin sill is almost in all localities, rudely prismatic: the prisms being perpendicular to the planes of the including strata, and not regularly cross-jointed. It appeared in some instances (Miner's bridge) that the lower part of the rock was finer in the grain than the other parts, more jointed and less prismatic, but the latter circumstance is reversed at the High force.

Considerable chemical changes are produced by the trap rock on the limestone, gritstone, and shale, with which it comes in contact or proximity. Not far from the south bank of the Tees, about Unthank and Holwick, and in the river at Winch bridge, sandstone and shale lying under the Whin, are greatly indurated, bleached, and full of joints; near the High force some portions of gritstone appear (included?) in contact with the Whin, are bleached in a singular manner, rendered very brittle and full of cracks. Shale or plate is so much altered at the High force in the relations of the joints, that most persons mistake a part of the prismatic masses really composed of

metamorphic shale, for trap, and suppose the latter to rest on limestone. The true series is as follows, proceeding downwards; (Diag. No. 12.)

Several beds of common dark limestone, with white shells and corals.

The limestone here seen below the Whin, does not present any remarkable characters implying the action of heat, (unless we except the pyritous layer at top), but in the grand precipice of White force, the limestone which supports the trap, and is in contact with it, is converted to granular limestone, and plate is greatly indurated.

The beds above the Whin sill are also locally much changed in structnre, texture, and aspect. The effects of heat are in some places more traceable above the Whin than under it; but they are no where important unless the mass of whin be considerable. Below Winch bridge, the Whin is thin, and limestone lying over it, (Tyne bottom limestone of Forster), is only partially affected. About half a mile below Winch bridge, I found plates alternating with three limestones ; the lower one was black within, and compact, the middle one was black and granular, both rather prismatized, the upper one blue, and partially pyritous; Professor Sedgwick found, near this locality, more decided effects of heat. (Camb. Trans. Vol. II, p. 171.)

The limestone lying above the Whin, in a circle round Caldron snout, is greatly changed from its usual aspect. The Tyne bottom limestone alluded to is, in this district, generally of a dark blue or blackish colour, and close texture, occasionally, however, containing crinoidal fragments and columns. On Widdybank scar, which seems about the point of maximum thickness of the Whin, this limestone is a

loosely aggregated mass of granular carbonate of lime, resembling coarse white sand. These calcareous sparry grains seem to be obscurely dodecahedral, and amongst them lie a few sparry crinoidal columns. As we recede from this centre, the limestone resumes by slow degrees somewhat of its ordinary character, passing through stages of white, bluish, and blue granular limestone, which grows more and more compact, darker, and harder, till its ordinary aspect is restored within a mile in the directions where the Whin is rapidly attenuated, but at greater distances where its thickness remains considerable. Under the same Widdy bank scars Professor Sedgwick ascertained the production of olive brown, or green garnets, in the cells of the anomalous fragmentary bed which supports the trap.

Since it is about Caldron snout that the Whin exhibits most thickness, and the greatest effects of heat in the rocks both above and below, it seems a plausible hypothesis to consider it as having been raised up through an opening in the lower limestone rocks at this place, and to have flowed hence in all directions for some distance, growing however very thin to the west, and vanishing totally to the south. The long ridge of basalt which extends down Teesdale, from Caldron snout to Lonton, may very probably indicate the line of a volcanic fissure, communicating with the presumed great opening at Caldron snout. Other such openings and fissures, along the line from Brampton to Belford, will explain the general conformity of the Whin sill to the limestone series.

But we may now inquire what proofs occur of mechanical disturbance produced by the eruption of the basaltic lava? On this subject Professor Sedgwick has left little to add to his very satisfactory description of the phenomena at White force, (Cronkley scar), in Widdybank scar, and near Lonton; I shall therefore adopt his descriptions, merely adding one additional instance, not visible when he made his

survey.

Lonton. Here, in the Lune, portions of the subjacent hazle are