56 



nearly all the principal mines of this whole coal field — It is, that the 

 main body of the coal lies either in direct contact with the primary- 

 rocks, or closely contiguous to them. This furnishes a highly im- 

 portant guide, or in fact the only unerring one, in the prosecution of 

 new or intricate workings ; for, it suggests the utility of making 

 the primary rock the object towards which the mining should be 

 pursued whenever a difficulty occurs in regaining the coal displaced 

 by a fault ; for, if we are surrounded by sandstone, we may be 

 sure that the chief deposite of the coal is to be reached by penetrating 

 across the strata towards the subjacent granitic floor. 



The sandstones being all of them nothing more than the debris 

 of the subjacent primary strata recemented, they resemble them 

 occasionally so much as to render it difficult to the inexperienced to 

 distinguish the two classes of rock ; though the discrimination in 

 this case is all important. The aid of a pocket magnifier will de- 

 tect a less angular character in the materials of the sandstones 

 than in those of the primary rock. 



The general range or longitudinal direction of the coal field, or 

 what is the same thing, the line of bearing of the outcrop of the 

 coal, is nearly N. N. E. and S. S. W. The structure of the coal 

 field is that of a true but very oblong basin, composed of a thick 

 series of variously constituted sandstones super-imposed upon two 

 or three seams of bituminous coal, themselves resting almost imme- 

 diately in contact with the surface of the primary rocks of the sur- 

 rounding region. Wherever the eastern boundary of the coal field 

 has been traced, the beds of gneiss or stratified granite are seen to 

 dip apparently with considerable regularity to the west, or beneath 

 the coal, and in like manner upon the opposite or western edge of 

 the tract, the same rock is known to have a general dip under the 

 coal to the east ; suggesting the notion, at first sight, that the strata 

 pass horizontally under the middle of the coal field. That this is not 

 necessarily the case, will appear presently. 



The usual angle of the primary strata to the horizon is about 

 25 or 30 degrees. Between the city of Richmond and the edge of 

 the coal field at Tuckahoe pursuing a progress westward, the dip of 

 the strata for the first several miles, is at a gentle angle to the E. 

 and N. E. varying sometimes to the north. Near the coal, the 

 point towards which the strata are dipping, is more nearly the N. 

 W. or occasionally more west. A similar change of dip is seen in 

 passing over the edges of the strata on the eastern side of the coal 



