Yorkshire. There is, generally speaking, a considerable difference in the external characters of these two beds; but at Alderley edge some parts of the red rock, containing mica, bear a greater resemblance to the shale than I have found in any other situation where I have examined it.
Mr. Whitehurst observed that a species of grit-stone, which he denominates the Mill-stone grit, is found under the coal, but never over it. This bed of rock, which in some situations is not less than 140 yards in thickness, varies in quality from a coarse-grained grit, approaching to a breccia, to a fine-grained siliceous sandstone. Some varieties of it are red, and bear a greater resemblance to the sand-rock of Lancashire and Cheshire than the red shale, which lies beneath the mill-stone grit.
It is not improbable that in distant places the same stratum may assume different characters, particularly when it belongs to that class of rocks which have been considered by geologists as mechanically deposited by the action of the tides. Strata thus formed may reasonably be supposed to alter with the materials of which they were made; materials that have been washed from the different parts of extensive ranges of mountains variously composed. I am not aware that this view of the subject has before been taken by geologists, although we may thus account for the gradual transition of rocks into one another, and may often give a more natural solution of the sudden changes we observe, than by the supposition of faults, of whose existence some evidence should always be given, independent of the difficulties which their admission would explain.
In thus comparing the geognostic position of the Red sandstone with that of the Mill-stone grit, I do not wish to advance any opinion of my own as to their identity, but merely to direct the attention of future enquirers to this subject.
- For the account of Cobalt ore contained in the Red rock at this place vide Monthly Magazine, February, 1811.