internal Structure of the Magnesian Limestone. 39 



formations. This has been, and perhaps still continues to be, a prolific cause 

 of error; and has often led geologists to confound the conglomerates which 

 are inferior to the great carboniferous order with those which are superior to 

 it.— 2ndly. When deposits like the new red sandstone rest unconformably 

 upon the inclined beds of the older formations, the fact demonstrates that 

 these beds were partially consolidated and mechanically tilted out of their ori- 

 ginal position before the existence of any part of the overlying mass. But we 

 have no right to assume, nor is there any reason to believe, that such disturb- 

 ing forces either acted uniformly or simultaneously throughout the world. 

 Formations which in one country are unconformable, may in another be 

 parallel to each other, and so intimately connected as to appear the production 

 of one epoch *. It is perhaps on this account that D' Aubuisson and some other 

 continental geologists have grouped the new red sandstone with the beds of 

 grit subordinate to the coal-measures. One who had formed his notions of 

 arrangement from the sections exhibited in the south-western coal districts of 

 England, would never have thought of such a classification. — 3rdly. When 

 two formations are unconformable, they are separated from each other by a 

 period of time to which we can assign no definite limit. It is impossible to 

 form even the shadow of a conjecture respecting the interval which elapsed 

 between the commencement of the great dislocations of the coal-measures and 

 the completion of the next superior deposit of magnesian limestone. And not 

 only was this period indefinite, but the mechanical agents which produced the 

 deposit appear to have operated with every possible variety of modification. 

 We may therefore expect to find near the lower part of the group of the new 

 red sandstone many important phsenomena in one country, of which we have 

 no example in another. This remark may perhaps explain the enormous de- 

 velopment in some of the Alpine regions, of a calcareous formation agreeing 

 in its great relations with our magnesian limestone. In the same way we may 



* It is perhaps unnecessary to accumulate proofs of this assertion. The position of the chalk 

 and other superior strata in the Isle of Wight and the Isle of Purbeck offer a well known ex- 

 ample of local disturbing forces, which operated on a great scale during the recent epoch of the 

 tertiary deposits. In a considerable part of the Jura-chain, a system of beds composed of salifer- 

 ous marls and gryphite limestone is unconformable to the superior oolitic formations. Here, 

 therefore, we have the indications of a catastrophe immediately subsequent to the deposition of 

 the lias, of which we have, I believe, no trace in this island. (See the memoir of M, Charbaut 

 on the Jura.chain, Annales des Mines, 1826.) Lastly, in South Wales, the old red sandstone 

 graduates into grauwacke, and is probably conformable to it. But in the northern counties of 

 England the same formations are unconformable, and are as perfectly distinguished from each 

 other as the dolomitic conglomerates of the western coal fields are from the rocks on which they 

 rest. 



