GEOLOGY OF DERBYSHIRE. 217 



4. Magnesian limestone 



5. Lower red sandstone. 

 Carboniferous system — 6. Coal measures 



7. Millstone grit 



8. Limestone shale 



9. Mountain limestone. 

 Igneous Rocks. 



A basaltic rock, commonly called toadstone, is found associated with 

 the mountain limestone. 



Superficial Accumulations or Diluvium. 



In the northern part of the district, there is little that would come 

 strictly under the term diluvium. There are proofs of great denuda- 

 tions having taken place, not only in the valleys which have been 

 worn, as it were, in the solid rocks, but more extensively over large 

 tracts of the more elevated surface. We know not, indeed, how we 

 can avoid supposing the shale and gritstone, and possibly the coal 

 measures, to have been once continuous over the district where now 

 the limestone is at the surface : but the stripping off of these, and 

 probably the denudation of parts of the limestone itself, belonged 

 to periods far anterior to that in which was formed what we now 

 term diluvium. Boulders of gritstone may occasionally be met 

 with within the limestone district ; but almost always in the val- 

 leys, where existing causes may possibly have carried them. The 

 flanks of the hills, too, are sometimes covered with what the miners 

 call clay and boulders ; but this is generally the debris of the neigh- 

 bouring hills, or the result of the decomposition of the rocks on 

 which it rests. No pebbles that have travelled from a distance are 

 ever found, so far as I am aware, on the hills of limestone or grit- 

 stone, or in the valleys among them. Boulders* of basalt or green- 

 stone, however, may sometimes be seen in the coal district, which 

 appear to have come from the north, as they differ in their mineral 

 character from any of the rocks in the neighbourhood or to the 

 south of it. On going south, however, towards the line of division 

 before mentioned, we find an immense accumulation of diluvium 

 piled over the comparatively low hills and covering the plain be- 

 yond. This diluvium is of several kinds. The hill at the back of 

 Ashbourn church is composed, to a considerable depth, of red clay, 



* Some of these were pointed out to me around Chesterfield, which were 

 two or three feet in diameter, and differed decidedly from any liasalt I ever 

 saw in Derbyshire. 



VOL. VIII., no. xxiv. 28 



