2d, The vast body of coal lying together, consisting of 10 beds. There are only 7 beds in the section, but the fourth coal is commonly reckoned three beds, and the lower coal two beds, there being a difference in the quality, with thin divisions in some places betwixt them. The whole thickness is 90 feet 2 inches. Some of the coal is of the open burning kind, but the great part is of the close burning quality, similar to the New castle coal, and breaks into small pieces. The coal abounds with inflammable air, and is liable to sponta neous ignition. In a great part of this coal-field the coals amount to only one-half of the thickness repre sented in the section ; but in the place where the sec tion is taken, the coals lie as if they had been cut through, and one-half slid over the top of the other. This singular coal-field is very limited in point of ex tent.
From an inspection of the four preceding sections it will be seen, that although the alternations of the strata are various, they may be referred to two kinds ; viz. sandstone and clay variously modified ; the only exception is the greenstone. There is no bed of lime stone in any of them ; this is however one of the strata found in coal-fields, and sometimes forms the roof of a coal. Beds of limestone are generally found in the low est series of the coal strata of a district.
With regard to coals termed workable or unwork able, this does not depend upon the thickness of a bed of coal, but upon the price obtained for it : so that a coal which is unworkable in one district, on account of its thinness, would be workable to advantage in another.
The greatest depth to which coal strata have been ascertained is 500 fathoms. This has been done in the Newcastle district, but it is not supposed that they go down to an indefinite depth, from the bason shape of coal-fields. It is with reason supposed, that the rocks upon which the Northumberland coal-field rests are a continuation of the metalliferous beds which form moun tains towards the county of Cumberland.
No coalfields of any extent are found to be absolute ly !Lit, although small portions of them may be so; they generally lie with an angle of inclination to the horizon ; they are, however, found at every various angle of in clination; viz. from the horizontal to the perpendicular line ; but the most common range of inclination is be twixt 3 and 11 degrees ; that is, from a rise of one foot perpendicular in twenty feet horizontal, to one foot perpendicular in five feet horizontal. In coal mining the angle of inclination is seldom mentioned, the com mon term for the inclination, or dip, being stated as one in five, one in ten, or one in twenty, according to the circumstances of the case. A coal is reckoned very flat, which lies at an inclination of about one in twenty ; and a coal is reckoned to have rather a great inclina tion, or what is termed a quick dip, when it lies at an inclination of about one in four. Coals having an in
clination of from one in three, to one in one, are termed half edge coals; and those which have a greater inclina tion than one in one are denominated edge coals.
As coals with their concomitant strata lie in a con forming situation, and parallel to each other, similar to the leaves of a book, and at an inclination with the ho-• rizon, having a cover of earthy alluvial matter in ge neral over them of considerable thickness, each of the strata meets the alluvial cover in the manner as repre sented, Plate CCCLXXXVIII. Fig. 1. where A is the surface line, B the under part of the alluvial cover C, and a, a. a, coals with their accompanying strata. This alluvial cover is found to be of various thickness, from a few inches to many fathoms; and it is this covering which creates the greatest difficulty in searching for coal and other minerals. It is in mineralogy to the strata which are underneath, what the flesh is to the bones in anatomy, when the covering is removed, the absolute structure is at once seen.
In describing the absolute forms or shapes of coal fields, it is necessary that the cover of earth above the strata be considered as removed, that every point. may be more easily comprehended ; and, to render the sub ject less intricate at first, we shall consider the coal fields as being of a regular shape, and without disloca tions.
In general, coal-fields have either the form of a cir cular or long elliptical bason, or are segments of these figures ; so that no extensive field of coal lies as to itself in the same plane, though a small portion of it may ap pear to be so. There are but few instances of the entire bason form, but this form we shall first explain, in or der to lead on to the more intricate varieties which oc cur. Plate CCCLXXXVIII. Fig. 2. represents the ho rizontal section of an elliptical coal-field, with the coals A and B rising to the surface in every direction, with their accompanying strata, and dipping to the centre pieces. They contain no inflammable air, but the car bonic acid gas is very abundant. It will be seen from inspecting the section, that sandstone abounds in the stratification, and that there are several considerable beds of red sandstone. It is necessary to remark, that the red sandstone of the cual•fields is altogether different from what is termed the Old red sandstone, in which no coals are supposed to exist. The red sandstone of the coal-fields is of a bluish colour, whereas the other sand stone is of a deep red, and much closer in the grain.