COAL : ITS STRUCTURE AND FORMATION. 361 



the absence or very small quantities of these substances 

 is, he contends, a serious argument against the generally 

 accepted vegetable nature of coals. Had such substances 

 ever been present, it is difficult to understand how they 

 could well have been removed by the solvent action of 

 water ; the amount of maceration in running water necessary 

 to eliminate these ash constituents, would probably involve 

 the destruction of all traces of organic structures. It is 

 always easy to fall back upon vaguely expressed chemical 

 changes as a convenient means of explaining certain facts, 

 but the point urged by Rigaud is one which should receive 

 attention at the hands of those qualified to deal with this 

 branch of the subject. The opaque black substance oc- 

 casionally met with in the cavities of coal tissue cells, is 

 regarded bv this writer as so much bituminous material 

 which has forced its way into the empty spaces. We have 

 abundant evidence, he suggests, as to the eruption of hydro- 

 carbons in past time in the bituminous shales, asphalte, and 

 other similar substances. If such outpourings took place on 

 a large scale on the floor of a lagoon or gulf into which 

 water carried down vegetable and mineral sediments, the 

 conditions would be favourable for the formation of beds of 

 coal, and somewhat analogous to those which at present 

 obtain in the pitch lake of Trinidad. The author of this 

 latest theory of bituminous eruptions, claims for it that a 

 critical examination of the facts and arguments should re- 

 sult in the verdict that it leads us a little nearer to the truth 

 than the previously accepted explanations of coal formation. 

 Any attempt to explain the manner in which coal has 

 been formed, must to a certain extent be founded on the 

 facts of microscopic structure. The occurrence of distinctly 

 marked impressions of plants on the surface of a piece of 

 coal is fairly common, and in the absence of any definite 

 markings imprinted on the surface, we can frequently detect 

 a fibrous structure in the dull layers of mineral charcoal or 

 mother of coal. In some places masses of coal are obviously 

 made up of flattened pieces of Sigillaria, Lepidodendron, 

 and other plants. Such instances are figured in Goeppert's 

 famous dissertation on the structure and formation of coal. 



