Geological Formations of Derbyshire.—Before we proceed on our walk, let us sit down awhile on this mossy bank, beneath the magnificent knoll of trees that here overshadows the river, and, by a reference to the geological table (p. 31) and the geological map, obtain a clear idea of the nature and succession of the strata around us; in other words, the order of superposition of the deposits of Derbyshire.
1. Lowermost. A bed of compact Basalt or Trap (in Derbyshire called Toadstone), of uncertain thickness and extent. This rock consists of mineral matter that has undergone complete fusion, and been erupted from the profound depths of some internal source of intense heat (see Wond. pp. 684, 848). The upheaving force thus put in action having been unequally exerted in different places, the superincumbent beds of limestone have been protruded in dome-shaped masses through the upper strata; and are now seen bent and curved, forming what is termed arched stratification.[802]
[802] My excellent friend, the late Robert Bakewell, Esq., was the first geologist who correctly explained the phenomena here described. See his Introduction to Geology, 5th edit. p. 147.
Vesicular or amygdaloidal Toadstone. This partakes more of the character of scoriæ, being full of little cells or cavities formed by air bubbles; when these cavities are filled with other mineral matter, as is often the case, the rock is termed amygdaloidal. This bed of Trap was formerly considered to be distinct from the lowermost; but it is now supposed, and with much probability, that all the masses of igneous rock that pierce, or are intercalated with, the sedimentary strata, have sprung from one common source, and are but lateral protrusions from some grand mass of erupted materials.[803]
[803] See an analogous example, Wond. p. 857.
2. Limestones with intervening layers of clay, and Magnesian limestones, or Dunstones.
3. Alternation of Limestone and Shale. Many of these limestones abound in organic remains; and it is in this group that the ornamental marbles of Derbyshire are comprised. The upper beds are generally of a slaty texture, and contain layers and nodules of chert, which often afford exquisite siliceous casts of the stems of Crinoidea (pulley-stones, see p. 284; and Wond. p. 650), and shells; white chert or porcelain-stone, and black jasper or flinty slate also occur in these beds.
4. Millstone Grit and Shale, and coarse sandstones; these form the subsoil of the principal heights of the mountain ranges, their sterile soil supporting only a covering of ling and heath.
5. Coal Measures; consisting of beds of Coal, with intervening layers of shale, clay, and ironstone (see [p. 80]).