of the bason C. Fig. S. is a section of the bason in the line DE. It is evident fl :on an inspection of the figures that the strata in the line FG 2 ) have a greater clip than the strata in the line DE, because they reach the cen tre of the basun in a shorter horizontal distance ; hence, if the bason were forty times longer than it is broad, the dip of the strata from D to C, and from E to C, would be vet y flat when compared with the dip from F to C, and from G to C, which %%mild be comparative ly great. here we conceive the bason to be of a regu lar elliptic form, but we find in general that the trans verse line DE does not divide the bason equally, but unequally, as represented in Fig. 4. where it is evident that the dip of the strata from H to C is much greater than the dip 1'1 om I to C, so that while the coals from C to I are flat, having a moderate rise, the coals From C to H have a great rise, and are frequently of the class of edge coals. A right understanding of these plans and sections will make the other figures of a coal-field and segments of the bason more easily comprehended.
The next form or shape of a coal-field is that which is semi-elliptical, as represented by the horizontal sec tion (Plate CCCLXXXVIII. Fig. 5 ) where A and B are coals with their accompanying strata, having their line of bearing in the direction C. D, and the line of dip from E to F, these coals are cut in the line of dip by a natural fissure in the strata C. D ; hence, in prosecu ting the working of the coals to the dip, they are lost, and in place of coal, nothing but stone, or the strata ac companying coal, are to be found : the nature of this fissure or obstruction will be explained afterwards.
These forms of coal-fields now described, or modi fications of these forms, are what we find the most com mon. There arc, however, instances of the inverted bason form, and of inverted segments of it; but these • are very rare in comparison with the other. Fig. 6. is the vertical section of this form, A, B, C, represents the coals dipping in every direction to F and G.
The cover of earth which lies above the strata is of two distinct kinds, both of which are alluvial, the one older than the other, viz.
1. The recent alluvial cover.
2. The old alluvial cover.
The recent alluvial cover is found by the sides of rivers and arms of the sea, in general very little elevated a bove the surface of the waters which evidently formed them.
The old alluvial cover succeeds the recent alluvial cover, is in general more elevated, and extends to the tops of the mountains ; there, however, it is found very thin. It is in the intermediate moderately rising ground where it is found of the greatest depth.
The recent alluvial cover we have found to be in some instances nearly 100 feet, and the old alluvial 180 feet in depth, above the rock.
The recent alluvial cover is generally composed of soil, clay, wet sandy loam, sludge, and sometimes beds of peat earth, also beds of gravel ; and in digging through it, trunks of large trees and brushwood are frequently found, with beds of sand containing great varieties of sea shells ; adjoining the arms of the sea, the remains of marine animals from the largest to the smallest size are also occasionally found. In the alluvial cover, by the sides of quick running rivers in coal districts, pieces of coal and of the accompanying strata abound, but adjoining slow running rivers and arms of the sea, these fragments arc seldom to be found, excepting when brought there accidentally.
The old alluvial cover is commonly composed of clay intimately mixed with sand, small rounded stones, and frequently with boulder stones, from a few pounds to several tons in weight ; it is quite impervious to water, and is termed till by agriculturists. In examining this cover when broken up, it is found to contain fragments of almost all the strata of the adjoining coal-field, such as sandstones, slate-clay, ironstones, and coal, the largest only a few inches in size, the angles sharp, which show they had not suffered by attrition. It is remarkable that no remains of trees or plants are to be found in this earth, nor any remains of marine animals or shells, though immediately adjoining the recent alluvial soil, where they are so abundant. This cover has some times beds of sand in it, and at the foot of mountains is mixed with many fragments of the mountain rocks.
As to the general situation of coal-fields with their accompanying strata, they are found, I. Under the level of the sea.
2. Above the level of the sea, moderately elevated.
3. Considerably elevated above the level of the sea.
Having considered the shape or form of coal-fields as uniform and entire, we have now to describe the disk cations and ruptures of the strata, for it is found in practice, that in place of a regular figure, the bason form is broken, dislocated, and deranged, forming seg ments of every various form, the dislocations, extend ing from a quarter of an inch to upwards of 600 feet, in a perpendicular or sloping direction, or separated horizontally from the breadth of an inch to upwards of 100 feet : It is these dislocations, which render the searching for coal, and the operations connected with the working of it, so difficult and intricate, while at the same time they are frequently of the greatest benefit.