valuable deposits of argillaceous iron ore which, existing in immediate contact with most excellent coal, have given to Wednesbury so distinguished a place among the iron manufactories of this kingdom, may be added the occurrence of a peculiar species of the ore called Blond metal, and which after being smelted, is used for the making of a variety of tools. The coal in this neighbourhood, is reckoned the best in the kingdom for the smith's forge: it is found in beds of from three to fourteen feet in thickness. Wolverhampton, another extensive manufacturing town in this county, owes its celebrity to the favourable position in which it is placed with reference to subterranean riches, and inland navigation. Situated nearly in the centre of the kingdom, in the midst of the most productive coal and iron mines, and having a free and easy access to the great rivers Thames, Severn, Trent, and Mersey, by means of the different canals which surround it, every opportunity is afforded of conveying and receiving materials and merchandise.*

argillaceous carbonates of iron, from which by far the greatest proportion of our British iron is smelted, is printed in Brewster's Edinburgh Journal for 1827-8, from the pen of Dr. Colquhoun.

Pitt's Staffordshire, p. 171.

CHAPTER VIII.

DISLOCATIONS OF STRATA.

Common occurrence of fissured strata-Longmire's theory of veins, dykes, rents, slips, &c.-How characterised—Up-throw and down-throw dykes-Section of fractured coal measures at Jarron-Enormous disturbances produced by faults-Great trap dyke of Yorkshire and Durham-The ninety-fathom dyke of Northumberland—The seventy-yards Whin dyke-The "great Derbyshire denudation" of Farey-Nonconformity of overlying and subjacent masses-Supposed igneous origin of trap or basaltic dykes-Advantages of those dislocations misnamed "faults"-Professor Buckland's observations.

ALTHOUGH several allusions have already been made to the subject of this Chapter, it is of too interesting a nature, in every point of view, to be merged in merely incidental notices. From what has been said already, it will have appeared, that besides the divisions of strata into different substances, often repeated after certain intervals, and generally extending in parallel series, variously inclined through the coal measures, there are certain fissures or fractures often nearly vertical, stretching through the whole

mass in a very singular manner, and betokening a violent upheaving or subsidence, and consequent separation of formerly continuous or adjacent portions. These rents, which every one must have remarked on a small scale in almost every stone quarry, and which traverse the coal formation in every district, are not only striking objects of inquiry to the geologist, but of vast importance in mining operations; and although generally termed faults, they are in reality of immense benefit in our colliery operations.

In the "Annals of Philosophy" for the year 1815, there is an elaborate essay on the shapes, dimensions, and positions of the spaces in the earth which are called Rents, and the arrangement of the matter in them, by Mr. J. B. Longmire, of Kendal. The object of the writer is to prove that metallic veins, dykes, slips, and other rents, in the internal part of the earth, were formed when it was passing from a fluid to a solid state, and are owing to an unequal contraction of its matter; and that the phenomena of stratification and formations, in some points of view, as well as the features of the earth at its surface, are effects of the same cause. Most of these fissures, particularly those supposed to have been the earliest formed, appear to have been filled by some of the matter at their sides being forced into them while yet in a fluid state, by the pressure of the superincumbent mass others appear to have been filled with matter that, at a later period, entered them either in a fluid state, as greenstone, basalt, &c., or in a solid state, as gravel, sand, and clay, generally mingled with fragments of the adjoining strata. The mineralogical compounds of the first class, varying in character from the distinct granitoidal crystalliza

MR. LONGMIRE'S THEORY.

159

tion of their original base, to the compact basalt, exhibiting but obscure traces of granular texture, are frequently met with in these subterranean clefts, as well as connecting with the various strata in overlying or proximate masses, and sometimes even intervening the earth. These masses, denominated by geologists trap rocks, and which are generally allowed to be of volcanic origin, have afforded no small portion of the ammunition which has been expended in the disputes between those who assign an aqueous, and those who contend for the igneous, formation of the strata composing the present crust of our planet.

But, to return to rents: these spaces are divided by Mr. Longmire into ovalar, cylindrical, straight and bended-tabular shapes. These latter are the most ordinarily occurring dislocations, and when found to contain earthy tabular masses, and metallic and earthy crystals, are known by the appellations of slips, dykes, shifts, lodes, troubles, and faults. They are denominated slips, by some geological writers, because the strata on one of their sides have slipped from those on the other, and fallen below them. They are known at Whitehaven and elsewhere by the name of dykes, because they divide the seams, or bands of coal, as they are sometimes called, into fields; and they are called up-throw or down-throw dykes, as the edge of the strata appears to an observer to be higher or lower in regard to his own position. They are called shifts in some parts of England and Scotland, as they are considered by the majority of miners in these parts to have shifted the strata on their sides. In Cornwall, they are denominated cross lodes, or, when round or ovalar, pipe veins; and in some parts of the kingdom they are called

troubles, or faults, from their troubling, or putting to fault, the pitmen; or the latter term may have arisen from the supposition that the rents have been occasioned from something faulty in the aggregation of the matter of the rocks themselves. The subjoined diagram, which will strikingly illustrate the dislocaFig. 18.

tions in question, is from a splendid section, on a large scale, presented by Mr. Buddle to the Natural History Society of Newcastle-upon-Tyne, in which neighbourhood (at Jarrow) the portion of coal measures thus singularly broken up, occurs.

We mentioned in the preceding Chapter, that the change of level in the same strata sometimes exceeds 500 feet. This amazing dislocation, indeed, appears almost trifling, when compared with the great south slip in the Clackmannanshire coal field, which, according to Mr. Bald, throws down the strata no less than 1230 feet; the north slip in the same field throwing it down 700 feet. These fissures extend from the surface of the strata, or rock-head, sometimes to an unknown depth; they vary in width from a fraction of an inch to four or five yards, or more. Their descent is sometimes nearly vertical, but more