54 



OUTLINES OF GEOLOGST. 



found phosphate of lime, fluor spar, and sulphate of baryta. 

 These minerals are confined to the millstone grit. 



127. The coal measures are composed of regularly strati- 

 fied rocks, the most important of which are : 



1. White or gray sand-stone ; 



2. Psammite, of white or gray colour ; 



3. Psammitic conglomerate ; 



4. Shale, often bituminous; 



5. Shaley clay ; 



6. Coal, bituminous and anthracite ; 



7. Iron stone, an argillaceous carbonate of iron. 



In addition, the millstone grit often alternates with the coal 

 measures, as does the limestone of the succeeding group. 



123. The coal measures sometimes overlie the millstone 

 grit and shale, and sometimes alternate with the carbonifer- 

 ous limestone ; hence their formation may be of any date 

 between the triasic, and the old red sand-stone. True coal 

 is also found in both the last-named groups, but never as yet 

 of sufficient thickness to be worthy of pursuit. 



129. Coal occurs in regular strata, co-extensive with the 

 other rocks of the formation, and with them appears to have 

 been deposited in basins of an extent less than any of the 

 formations of the supermedial order. Each basin, or coal 

 field, contains many beds of coal, which are separated from 

 each other by strata, often numerous, of the other rocks 

 These beds vary from many feet in thickness, to mere seams. 

 One bed in the coal field of Liege is 60 feet thick, another in 

 Scotland 56. A thickness of 6 or 8 feet is most convenient 

 for working ; and it will not defray the expense of mining, if 

 less than 18 inches. 



130. Coal affords distinct evidence of vegetable origin, in 

 fossil charcoal, and vegetable impressions. The organic re- 

 mains of the group are numerous, and almost wholly vege" 

 table. The few animal remains are the shells of fresh water 

 molluscse. 



