Geology of Coalbrook Dale. 465 



also several species of Unio — the first trace of animal life we meet with in the coal-measure. The 

 same Sti^maria is also very common in the little flint-coal and sandstone above the crawstone. 



The characters of this series, therefore, apparently indicate a period of considerable turbulence, 

 or a sudden and violent change in the condition of the land succeeding a long period of repose, 

 during which the carboniferous limestone had been formed. 



In the lower strata the thick sandstones and conglomerates, the great and rapid changes in se- 

 veral beds at very short distances, and the local occurrence of one bed and the absence of others 

 at greater distances, all indicate a period of much disturbance and sudden changes ; but these 

 signs of varying velocities gradually diminish as we ascend the series. Such effects we are natu- 

 rally led to expect, if we consider the great difference between the characters of the coal-measures 

 and those of the carboniferous limestone. In the latter, we have every indication of a formation 

 gradually accumulated in the ocean during a lengthened period, whereas the structure of the for- 

 mer denotes the proximity of dry land, and bears evidence of violent operations by which one 

 portion of the bed of the ocean became dry land, while another was partially raised and converted 

 into an extensive estuary, receiving the detritus borne down by the rivers intersecting the newly- 

 exposed land. The abrasion of the recently elevated tracts, then but barely covered with soil, may 

 account for the prevalence of thick, unproductive, coarse sandstones in this lower division, while 

 the presence of a few coal seams, and of the crawstone with its well-preserved specimens of de- 

 licate Stigmaria, higher in the series, may have arisen from a more abundant growth of plants 

 during a period of calm, succeeding the first effect of the change of level. The recent origin of the 

 estuary and the violent action attendant upon the transport of the conglomerate, would account for 

 the absence, in the lowermost beds, of marine Testacea. 



The next series of strata, extending from the clod-coal to the Penneystone, consists, in the 

 lower portion, chiefly of thick beds of shale, containing large quantities of finely-preserved 

 vegetable remains, and alternating near the bottom very frequently with seams of coal. In 

 the south of the coal-field some of the shales pass into sandstones, which always inclose an 

 increased number of plants in passing from a fine to a coarser grain. This point appears to 

 me to be important, and to prove either a greater supply of vegetable remains from a body of 

 water more widely extended and of greater transporting power, or a more complete decay in the 

 plants from the slower accumulation of the fine-grained sandstone. The coal-beds are far more 

 persistent in this than in other portions of the coal-measures. Each bed is generally underlaid by 

 a thin seam of indurated clay, containing few or no fossils ; whereas the overlying stratum suc- 

 ceeding the few inches of fine bituminous shale, immediately incumbent on the coal, is coarser and 

 thicker and abounds in vegetable casts and impressions, which decrease rapidly in number as the 

 distance from the coal increases. From these facts, I would infer, that the lower clay and shale 

 were gently washed down during short periods of calm, and of rapid vegetation, and that the 

 overlying coarser silt and sand were accumulated by increased volume and violence in the current, 

 consequent upon the floods which originated the coal seams. It nevertheless appears probable, that 

 no lengthened period elapsed between the uprooting of the plants and their entombment, for the 

 casts and impressions in some of the roofs of the coal-bed, exhibit perfectly the delicate external 

 organization of several genera of ferns and Lycopodiceae, and sometimes retain a portion of their 

 vegetable tissue. The period, therefore, during which this series was deposited, seems to have 

 been one of repeated but not violent changes. 



Thus far, in the portion of the lowest measures which we have reviewed, the unsettled state of 

 the land and water appears to have checked the increase of animal life. We now, however, ar- 



