126 LECTURES 



following up signs of coal, unless we are sure we are in tlie region of 

 the carboniferous strata. 



The carboniferous strata are subdivided into two very distinct groups, 

 representing, of course, distiuctsubdivisionsof the carboniferous period. 

 These are called lower and upper carboniferous, or the mountain lime- 

 stone, and the "coal measures." The former are mostly limestone, 

 the latter mostly shales and sandstone ; the one mostly of marine 

 origin, the other mostly fresh water ; the fossils of the one are mostly 

 marine animals, of the other terrestrial vegetation. I shall confine 

 myself entirely to the latter, or the true ^^coal measures," as they are 

 called, from the fact that ninety-nine hundredths of all the coal in the 

 world are found in them. 



You will observe, then, that I have taken for my subject one-half 

 of the carboniferous period. The carboniferous is itself but one of the 

 four great subdivisions of the palre'zoic period^ and the palfBzoic period, 

 in its turn, only one of the four great epochs, exclusive of the present, 

 into which the history of our earth is divided. You see, then, that 

 the period of which I wish to give you a rapid sketch is less than 

 one-thirtieth part of the recorded history of the earth ; yet the average 

 thickness of these strata is about 3,000 or 4,000 feet. In Wales they 

 are 12,000 feet thick, and in Nova Scotia nearly 15,000. If, then, 

 thickness of strata represent length of time, how great must be the 

 lapse of time represented by the coal measures. 



Such being the enormous thickness of the coal measures, it neces- 

 sarily follows that but a very small proportion of the mass consists of 

 coal. The coal strata consist of thick beds of limestone, sandstone, 

 ironstone, and shale, containing thin seams of coal, and this alterna- 

 tion sometimes many times repeated in the same locality ; the whole 

 forming a series like the sheets of a ream of paper, arranged in no 

 discoverable rational order, but indiscriminately alternating. The 

 seam of coal will sometimes be covered with a stratum of limestone, 

 sometimes of standstone, and sometimes of shale ; although it rarely 

 happens that the sandstone or limestone comes directly in contact 

 with the coal ; but is generally separated by a stratum, sometimes 

 very thin, of shale or slate. In fact a stratum of clay or fine mud 

 rock both underlies and overlies each seam. Below it forms the "fire 

 clay," and above the black slate of the miners. 



F.ff. 1. J have said that the order is various in different parts of 

 the same alternating series ; but in every part of the same 

 coal field the alternation is the same for the same part of 

 the series. In other words, each stratum is generally 

 horizontally extended over the whole coal field in a con- 

 tinuous sheet, so that each seam is accompanied by the 

 same strata above and below. This is a fact of great 

 importance, as it affords the readiest means of determining 

 the identity of individual coal seams. 



Coal strata, like all other sedimentary deposits, were at 

 the time of formation horizontal, or nearly so. Sometimes 

 they are found nearly in this their original position, as in 

 many of the coal fields of our own country. More generally this 

 original horizontality has been disturbed by igneous agency, and the 



