228 A POPULAR SKETCH OF THE 



detached portions of coal which are found separated from the regular 

 coal district, more especially the coal which is worked at Aldervvas- 

 ley, which lies, I believe, between the third and fourth gritstones of 

 Farey, and is, I am informed, the same as the Alton coal, which 

 certainly occupies that position near Ashover. The gritstones of 

 this group vary, in different portions of the same bed, from a coarse 

 conglomerate of small clear quartz pebbles, to an almost fine grain- 

 ed sandstone. Many portions are hard enough for millstones, and 

 almost all of them make an excellent material for railway blocks 

 and similar purposes. They are thick bedded universally ; and 

 sometimes so much so that, for this reason and on account of the 

 oblique lamination so prevalent in all sandstones, it is very difficult 

 even in large quarries to discover the real bedding and dip of the 

 rock. The oblique lamination shows, probably, the action of strong 

 currents, which depositing successive layers of sand on the side of a 

 slope, have caused the lamina of a bed to incline at a considerable 

 angle in one or more directions, while the bed itself, which is made 

 up of these lamina, may be horizontal or dip in another direction. 

 The thickness of the lowest grit, or that which is known in the 

 country as the millstone grit, being upwards of two hundred feet, 

 it is probable that the thickness of the rocks which are here classed 

 as the millstone grit group would be between four and five hundred 

 feet. 



Organic Remains of the Coal Measures and Millstone 

 Grit. 



These are both animal and vegetable, by far the greater portion 

 belonging to the latter class. The animal remains consist of shells 

 and fish. In some of the bands of ironstone, and occasionally in the 

 shales, there are found a great number of bivalve shells, which, as 

 they belong to the genus Unio, must have lived in freshwater. 

 There are several species, of some of which the individuals occur 

 in the greatest abundance, and are sometimes beautifully perfect and 

 well-preserved. On splitting open the masses of black shale which 

 lie upon the coal, and which may be found around a newly-sunk 

 shaft, teeth and scales of fish, as well as fragments of their bones, 

 may be found*, similar to those which are contained in other coal- 

 fields. The fish of the carboniferous system have been shown by 



" Mr. Atkinson, of Chesterfield pointed out the occurrence of these fossils 

 to me in the neighbourhood of that town. 



