128 



LECTURES 



dislocation of tlie strata, producing what are called faults, slips, or 

 troubles. In tlie accompanying figures, for instance, the strata have 



Fig. 7. 



Fig. 8. 



been displaced hj the elevation of one part of the field more than 

 another. This is not conspicuous on the surface, because all has been 

 cut down to one level by aqueous agencies. The supposed configura- 

 tion of surface immediately after such unequal elevation is represented 

 by the dotted outline ; the strong line represents the present configu- 

 ration of surface. All between these, therefore, represents the 

 amount of matter carried away by denuding agencies. These faults 

 occur very often in coal fields, and are a source of serious annoyance 

 to the miner. 



I have taken here the simplest case of dislocation. The difficulty 

 becomes very much greater when, instead of being horizontal, the 

 strata are highly and variously inclined. In such cases the skill and 

 knov^ledge of the geologist is often tasked to the utmost. 



I have said that while metallic veins extend indefinitely downwards, 

 coal scams for the most part are extended horizontally, or nearly so. 

 Sometimes, however, coal seams may appear, like metallic veins, to 

 extend downwards. This is the case in highly inclined and particu- 

 larly in vertical strata, as in the accompanying sketch of the anthracite 

 coal field of Pennsylvania. In such cases, however, as well as in 



Fig. 9. 



every other, it will be observed that the seams are strictly parallel 

 with the strata, that the strata have been elevated to a vertical posi- 

 tion by igneous agency, and the included coal seams have been raised 

 with them, still maintaining their relative position. 



The thickness of coal seams varies from a few lines to many feet ; 

 sometimes they exist as sheets as thin as paper, in others in masses 30 

 or 40 feet thick. A single seam of pure coal, however, is seldom more 

 than 6 or 8 feet thick. It is true that in France and in the anthracite 

 region of Pennsylvania they are said to occur 60 or 70 feet thick, or 

 even more, but upon close examination such mammoth seams will be 

 found to consist of two or more seams, separated by thin laminas of 

 slate ; too thin, however, to form a roof, and, therefore^ the several 

 seams are wrought together as one. 



The number of seams occurring in one locality and separated by 

 interstratified sandstone and shale is sometimes as great as one 

 hundred, and their aggregate thickness one hundred and fifty feet. 

 Enormous as is this mass of carbonaceous matter, it is but a small 

 fraction of the entire mass of the coal strata. The thickest and purest 



