88 



OUTLINES OF GEOLOGY. 



that these prevailing families, which at the present day are 

 mere herbs, or rarely aspire to the size of shrubs, exist in the 

 medial order of the most gigantic dimensions. 



195. The study of the manner of formation of coal is inti- 

 mately connected with the subject of vegetable remains. 



Carbonaceous matter is not limited to the coal measures, 

 but is found in every one of the orders of stratified rocks, as 

 well as in the alluvium and diluvium. 



(1.) Beds of turf form a part of alluvial formations, and 

 are caused by the growth and death of vegetables, which still 

 live and flourish in the immediate vicinity. These beds of- 

 ten contain fossil trees, in sufficient abundance to resemble 

 prostrate forests. The wood of these is partially altered, 

 the elements of the oleaginous and resinous parts having en- 

 tered into a new combination, namely, bitumen. 



(2.) Beds of turf are also covered and intermingled with 

 diluvium. The turf of these has undergone a chemical 

 change, and the imbedded trees have often passed into lignite 

 or brown coal. The bed of brown coal at Bovey-Heath- 

 field, is covered only by diluvium. 



(3.) Lignite, jet, and amber occur in the formations of the 

 superior order ; the two latter are of a vegetable origin as 

 unquestionable as the first. 



(4.) Thin seams of brown coal, often retaining a distinct 

 vegetable structure, occur in the chalk, but are more fre- 

 quent in the green and iron sand. Similar seams are found 

 in the oolitic group. In the triasic group true coal makes its 

 first appearance, although in thin seams of no value for 

 working. 



(5.) The anthracite and bituminous coal of the coal mea- 

 sures, are often intermingled with fossil charcoal, retaining 

 the ligneous structure. Vegetable impressions are abundant 

 in the shales contiguous to the coal, and although the pecu- 

 liar cubical fracture of bituminous coal, and the conchoidal 



