42 Featherstonhaugh^s Geological Report. 



There is another case of complication connected with the 

 inclined beds, which the student will easily see the great 

 importance of. Rocks whose planes are horizontal, or in any 

 manner parallel to each other, are called conformable, but it 

 frequently occurs that they are unconformable to each other. 

 Diagram No. 5 will give an example of this. Here the stra- 

 tum a may represent the new red sandstone (see the tabular 

 view) at the surface ; and as it most frequently, when found, 

 lies superimposed on the bituminous coal measures, it would 

 be consistent with practical and theoretical knowledge to dig 

 through it at N for coal, the true position of which may be at 6. 

 In this case, the miners, beginning midway of the diagram, 

 and sinking their shafts o a a o towards N, might come upon 

 the strata, e, /, g", /i, which lie at a great angle to a, and are 

 unconformable to it, having been tilted up into this inclination 

 before the horizontal beds, a, 6, c, c?, were deposited. All 

 this expense would be wasted by unskilful persons ; but an 

 experienced miner, acquainted with practical geology, and 

 understanding the reason of this deficiency of the intervening 

 beds, 6, c, 6?, beyond the point +, would sink through a further 

 towards S, and thus be rewarded by the discovery of 6, 

 containing coal veins in their regular position. 



The stratified inclined rocks comprehended in the Cam- 

 brian and Silurian systems of Europe, which have been 

 spoken of, have many of their proper equivalents in the 

 various formations which occur up the valley of the Potomac, 

 between the great falls of that river and the southeast edge 

 of the great Western coal field, and whose beds have been 

 hitherto alluded to by geological writers, as constituting the 

 transition and grauwacke rocks of the Alleghany ridges and 

 country parallel to them on the southeast. The inspection of 

 this valley is highly favorable to the acquisition of just views 

 respecting the geological structure of those numerous ridges, 

 sometimes continuous, sometimes interrupted, sometimes flex- 

 uous, and generally anticlinal. An accurate knowledge of their 



