H. C. Sargent — Clay-bands in the Crick Inlier. 411 



incapable of identification. The black colour is no doubt due to the 

 presence of ferrous sulphide. Minute crystals of selenite are abundant, 

 suggestive of the action of decomposing pyrites on carbonate of lime. 



It is not easy to explain the presence of these fossils in this limited 

 exposure, but their fragmentary condition and the entire absence of 

 carbonaceous matter in the bed seem to indicate a derivative origin. 

 One or two suggestions may perhaps be permitted. If this black 

 central portion of the bed represents a local shower of ash, following 

 a temporary cessation of the eruption of lava, the presence of fossils 

 would present no difficulty. On the other hand, cases could be cited 

 in which a rapidly cooling lava-flow has embedded remains without 

 entirely destroying them, and it seems not impossible that this may 

 have happened here.^ Whatever the solution, the great difference 

 between this portion of the bed and. its other exposures, in structure, 

 chemical composition, and appearance, would seem to indicate some 

 difference in origin. 



Another objection which may be raised against the hypothesis of 

 volcanic origin is the absence of alteration in the rock below the 

 clays. It must be borne in mind that such thin lava-flows would 

 cool very rapidly on the sea-floor, and in other parts of the county 

 very little and sometimes no alteration has been effected in the rock 

 in contact with beds of basalt much thicker than these clays. ^ 



Dr. Arnold-Bemrose has kindly called my attention to the fact that 

 the view set forth in this paper is not of recent origin. Thus, sixty 

 years ago Sir H. de la Beche wrote as follows : " Although there are 

 clays amid the limestones in the relative positions of the igneous rocks, 

 and some of these seem clearly little else than such rocks in a highly- 

 decomposed state, retaining the arrangement of their component 

 mineral substances, as, for example, at the isolated boss of limestone at 

 Crich [italics mine], ... it Avould scarcely be safe to conclude that 

 all lying nearly in the same general geological levels were so, 

 inasmuch as some of them may be clays of another character. Care 

 on this head is rendered necessary by finding a clay — a true under- 

 clay of the coal-measure kind — supporting a thin bed of impure coal 

 in the higher part of the limestone series near Matlock Bath." ^ 



It is an interesting circumstance that, in a quarry near Matlock 

 Bath, there is at the present time a bed of clay exposed, supporting 

 a thin bed of black shale with Pterinopecten papyraceus, and 

 occasionally a thin coal is found on the clay. There can be little 

 doubt that these are the beds referred to by Sir H. de la Beche. 

 I have examined this clay, and although in external appearance it 

 much resembles a fireclay, in microscopic structure it is quite 

 different and agrees very closely with the clays described above. 

 1 have little doubt that it is of igneous origin. The rock above is 

 thin-bedded, with thin partings of unmistakable detrital structure 

 between the beds, so that no doubt a shore-line was not far distant, 

 and thus the presence of the shale and coal is not so anomalous as it 

 otherwise would be. 



^ Cf. Lyell, Principles of Geology, 12th ed., vol. ii, p. 516, 1875. 

 2 Cf. Geol. Surv. Mem., North 'Derbyshire, 2nd ed., 1887, p. 20. 

 ^ Geological Observer, 2nd ed., 1853, p. 560. 



