244 



LYELL'S ELEMENTS OP GEOLOGY. 



Carboniferous Group Freshwater Strata. 



Beneath all these is the Old Red sandstone, which was for- 

 merly considered as part of the Carboniferous series ; but which, 

 now that its organic remains are better known, appears entitled 

 to rank as a distinct formation. 



As we proceed northwards from South Wales and Somerset- 

 shire to Yorkshire and the more northern counties, we find the 

 Carboniferous group beginning gradually to assume a new cha- 

 racter, there being first a slight intermixture of the Coal-measures 

 and Mountain limestone at their contact, and these alternations 

 taking place afterwards on a still greater scale. The Coal, in 

 Yorkshire, does not cease when we reach the Millstone-grit, 

 although it is there in diminished quantity ; and beneath that 

 grit is a complex deposit, 1000 feet thick, of limestones, alter- 

 nating with coal-bearing sandstones and shale, below which 

 comes the great mass of mountain limestone.* In Scotland we 

 observe a still wider departure from the type of the south of 

 England, the mixture of marine limestone with sandstone and 

 shale, containing coal, being more complete. 



The importance of the coal in England, considered economi- 

 cally, is greatly enhanced by the rich beds of iron-ore which 

 occur in tiie associated shales, and the contiguity as a flux to re- 

 duce the iron-ore to a metallic state.f 



It is now generally admitted, that all coal is of vegetable ori- 

 gin, the vegetable structure being still recognizable in many kinds 

 of coal, when slices thin enough to transmit light are obtained 

 and examined by the miscroscope. Impressions also of plants, 

 together with entire trunks of trees, are frequently met with in 

 the accompanying shale and sandstone ; leaves also, and small 

 branches, and fruits, occur in nodules of clay-ironstone, the in- 

 closed vegetable having served as a nucleus round which the 

 ferruginous matter, usually carbonate of iron, has concreted. 

 Some of the coal-measures are of freshwater origin, and many 

 have been formed in lakes, others seem to have been deposited 

 in estuaries, or at the mouths of rivers, in spaces alternately oc- 

 cupied by fresh and salt water. 



Thus a freshwater deposit, near Shrewsbury, has been ascer- 

 tained by Mr. Murchison to be the youngest member of the car- 

 boniferous series of that district, at the point where the coal-mea- 

 sures pass into the lower New Red formation. It consists of 

 shales and sandstones about 150 feet thick, with coal, and traces 



* Sedgwick, Geol. Trans., Second Series, vol. iv. ; and Phillips, Geol. of 

 Yorksh., part 2. 

 t Conybeare, Outlines, &c., p. 333. 



