Lign. 274. Section of Crich Hill; from A to B on the plan, [Lign. 273].
(Natural scale.)
By Reginald Neville Mantell.
| A, B.— | Strata of Millstone Grit, highly inclined, and dipping from the central axis. |
| Protrusion of the Mountain Limestone, on the summit of which is Crich Tower, 716 feet above the level of the river Derwent. | |
| Central mass of Trap, reached by a vertical shaft from above, and by a gallery on the side of the hill. |
It is therefore evident, that at Crich the strata No. 2 must have been forced up, and protruded through the strata No. 1, or they could not occupy their present position. We remarked, on ascending Holloway Hill, the great disturbance which the Millstone beds had sustained (see the vignette of this volume); and the shaft and gallery near Wakebridge ([p. 884]) disclosed the existence of a mass of basalt, or trap, of unknown extent, beneath the limestone, in the centre of the mountain; while the fissures and crevices, filled with metallic ores and spars, attest the action of intense heat, under great pressure. According to a survey made by my son, a section in the line marked A—B on the plan, [Lign. 273], presents the arrangement of the strata seen in [Lign. 274]: the heights are from actual admeasurements.
From the data thus obtained, we may construct an ideal section in illustration of these phenomena, as in the following diagram ([Lign. 275]).
Here then, as Mr. Bakewell forcibly observed, "we have cause and effect in conjunction." Here is the cooled and consolidated molten rock, whose expansive force elevated the horizontal strata of limestone, and forced them through the superincumbent beds of grit and sandstone. But this eruption must have taken place under great pressure, and at the bottom of the sea; for, had the phenomena been sub-aërial, the result would have been altogether of a different nature; and we should have had cooled lava streams, and not masses of basalt.
We have seen that the strata rise round and enfold this central nucleus of volcanic rock, displaying nearly hemispherical segments and curves. Now if we suppose a vertical transverse fissure across such a hill as that represented in the diagram ([Lign. 275]), the face of the remaining strata would be in every respect similar to that of the High Tor (see [Lign. 270], p. 876); namely, a mass of Trap, or toadstone, at the base, and a series of arched strata of limestone above; with fissures containing ores of lead, zinc, barytes, &c. and various kinds of spar.