MINERAL COAL. 87 



perhaps layers of limestone ; and then a Jiird bed of coal, 

 and so on. By such alternations the series is completed. 

 Immediately in the vicinity of the coal, the rock is generally 

 rather a shale than a. sandstone, and these shales are usually 

 full of impressions of leaves and stems of plants. The clay 

 shales are sometimes quite soft and earthy, and of a light 

 clay color ; but in most coal regions they are hard and firm, 

 with a brownish or black color, in the vicinity of the coal 

 layer. The sandstones are either of a grayish, bluish, or 

 reddish color. 



These various layers constituting coal beds, are some- 

 times nearly or quite horizontal in position, as in New Hol- 

 land and west of the Appalachians. They are very often 

 much tilted, dipping at various angles and sometimes verti- 

 cal, as is generally the case throughout central Pennsylvania ; 

 and in some cases the beds are raised in immense folds, as 

 the leaves of a book may be folded, by a sidewise pressure. 

 They are very commonly intersected by fractures, along 

 which the coal seam on one side is higher or lower than on 

 the other, owing to a dislocation, (then said to be faulted) ; 

 and miners working in a bed for a while, in such a case, 

 find it to terminate abruptly, and have to explore above or 

 below for its ■ continuation. These are points of great im- 

 portance in the mining of coal. 



There is no infallible indication of the presence of coal 

 distinguishable in the mineral nature of rocks ; for just such 

 rocks as are here described occur where no coal is to be 

 found, and where none is to be expected. The presence of 

 fossil leaves of ferns, and of plants having jointed stems or-a 

 scarred or embossed surface, in the shales or sandstone, is a 

 useful hint ; the discovery of the coal itself a much better 

 one. The geologist ascertains the absence of coal from a 

 region by examining the fossils in the rocks ; these fossils 

 being different in rocks of different ages, they indicate at 

 once whether the beds under investigation belong to what is 

 called the coal series. If they contain certain trilobites, 

 and other species which are found only in more ancient 

 rocks, there is no longer a doubt that coal is not to be ob- 

 tained in any workable quantities ; and he arrives at the 

 same conclusion if the remains are those of more recent 



What is said of the position of the beds ? How do the rocks indicate 

 whether coal is to be expeotrd in a region or not? 



