already noticed, and require no additional remarks here. The reader must be referred, for a fuller account of all these substances, to the topographical and mi- neralogical works in which they are described. But a few remarks on their present condition will not be useless. It is evident that the upper clays, such as those of the London district, are now in the same state in which they were deposited; and that, although they might have produced rocks, under other circumstances, they have never existed in that form since their deposition. This seems equally true of many of the inferior strata of the same nature; however difficult it maybe to explain why, in certain situations, the same material should have remained uncompacted into stone, while, in others, it has become shale. Yet the remarks in a former chapter have shown that many of these clays have once been in the state of rocks, which have been decomposed within the earth, so as to assume their present form. This, indeed, is proved in Cornwall, where the ordinary argillaceous schist is found con verted into clay at great depths, retaining the indica tions of its former state ; as are granite, gneiss, and the traps in innumerable situations. The same re marks may be made on the sands : but it would be merely to repeat the same reasoning; and I need only remind the reader of the proofs of extensive decom position among these, formerly given. Neither is it necessary to extend this reasoning to the marls ; as, in some cases, they are obviously in the state in which they were first formed; while, in others, it is equally apparent that they are the result of decomposition. But the term Marl has also been applied to a schistose rock, so as to have been the cause of much inconvenience. That of Marl-slate has particular-