internal Structure of the Magnesian Limestone. 65 



and that in some places they reposed upon, and seemed to pass into a yellow 

 incoherent coarse siliceous sand. It appeared, therefore, that those mecha- 

 nical agents, which in many parts of England had acted with such destructive 

 violence, had here operated upon the carboniferous strata much more feebly, 

 and only produced a number of irregular masses of drift sand, on which the 

 formation of yellow limestone was subsequently deposited. 



During the same year I had an opportunity of observing in a part of York- 

 shire, that the magnesian limestone rested upon a system of beds of very 

 peculiar character, which in some places resembled coarse millstone <^ni, and 

 in others had more the appearance of new red sandstone. As, however, I had 

 at that time no means of ascertaining the extent and continuity of this deposit, 

 and as I found that its upper surface was in some places unconformable to the 

 limestone which rested upon it, I erroneously concluded that it was a peculiar 

 formation of gritstone subordinate to the Yorkshire coal-field. 



Not long afterwards I became acquainted with Mr. Smith's geological map 

 of Yorkshire (which was published in the year 1821), and then, for the first 

 time, I saw the importance of the above-mentioned deposit in that county. It 

 is there shown to be coextensive with the magnesian limestone, from which it 

 follows that it must be unconformable to the coal-measures. Hence, notwith- 

 standing its mineralogical character, which in some places almost identifies it 

 with the inferior strata, it is absolutely necessary to separate it from them, 

 and to regard it as the first term of an entirely new series of deposits, of which 

 the magnesian limestone forms so prominent a part. To Mr. Smith, therefore, 

 belongs the honour of having added this member to our English secondary 

 formations. In classing it with the coal-measures he, however, deprived it of 

 its real importance : and Mr. Conybeare was, I believe, the first who published 

 an opinion that it was analogous to the rothe-todte-liegende ; and therefore 

 formed a new connecting link between the physical history of our own country 

 and that of the continent. 



During several subsequent visits to various portions of the escarpment of 

 the magnesian limestone, I had an opportunity of verifying many of the details 

 given in the geological map of Yorkshire, and of ascertaining- that the beds 

 of incoherent yellow sand which I had before observed in the county of Dur- 

 ham, are nearly coextensive with the limestone which rests upon them. As 

 the result of all these observations, it appears that, with a few interruptions, a 

 formation of sand and sandstone of variable structure and thickness may be 

 traced between the coal-measures and the magnesian limestone, from the 

 mouth of the Tyne to the confines of Derbyshire*. 



* In the geological map of the county of Durham no notice is taken of the lower red sandstone. 

 VOL. III. SECOND SERIES. K 



