GEOLOGY OF DERBYSHIRE. 223 



from a coal-measure gritstone. Its thickness likewise varies from a 

 few feet to two or three hundred, but in Derbyshire and Notting- 

 hamshire it can never amount to so much. Its fossils are few, and 

 when they do occur consist of coal plants. 



The rocks which lie immediately* below those of the new red 

 sandstone system, are a complicated and important mass of materi- 

 als, which from one of their most useful products are termed the 

 carboniferous group or system of rocks. 



The Carboniferous Group 



consists of limestone, shale, sandstone, coal, and ironstone, the dis- 

 position of which materials, when the group is examined generally, 

 is very variable. In Derbyshire, however, and over the southern 

 part of England, the order in which they occur is found sufficiently 

 constant to admit of the subdivision of the group into distinct parts. 

 This subdivision is, perhaps, most complete in Derbyshire itself, 

 where we find that the coal is confined to the upper portion of the 

 group, and the limestone to the lower, its middle parts being almost 

 entirely shale and sandstone, or gritstone. In determining these 

 divisions, however, we must always remember that there are no 

 hard and well defined lines between them existing in nature, but 

 that each portion passes into the other by almost insensible grada- 

 tions. The positive places, then, of each of our division lines must 

 be in a great measure arbitrary and determined by our convenience, 

 following as near as possible those marked out by Nature. 



Beginning with the highest portion of the group in Derbyshire, 

 we should find a series which may be represented by the following 

 list, in which the different sizes of the type represent the differences 

 in the relative importance of the beds : 



Shale 



Coal 



Shale, with ironstone 



Coal 



* The reader will always bear in mind that one rock may be below ano- 

 ther in the geological series, but that the country formed of the lower rock 

 will frequently be much higher above the level of the sea than the country 

 formed of the upper rock. This is owing to the dip, or inclined position of 

 the rocks, in consequence of which the same beds, which in one place form 

 lofty hills, gradually sink, till in a few miles, perhaps, they plunge under 

 others which form a low and level country. 



