(54 Rev. A. Sedgwick on the Geological Relations and 



Chapter II. — Internal Structure of the Formation of Magnesian 



Limestone, ^c. Sgc 



Havino- described in general terms the range and extent of tlie formation of 

 magnesian limestone, the external character of the country through which it 

 passes, and its relations to the strata of the carboniferous order, I now pro- 

 ceed to give some account of its internal structure, and of the composition of 

 its subordinate parts. In doing this, it appears necessary, for reasons already 

 stated, to regard it as a complex formation subordinate to the group of the 

 new red sandstone ; in which case it admits of the following natural subdi- 

 visions. 



1. Lower red sandstone. 



2. Marl-slate, associated with grey, thin-bedded, and nearly compact lime- 

 stone. 



2a. Variously coloured marls, with thin beds of compact and shelly limestone. 



3. A great deposit of yellow magnesian limestone; often cellular and earthy, 

 sometimes hard and crystalline. 



4. Lower red marl and gypsum. 



5. Grey thin-bedded limestone. 



6. Upper red sandstone. 



7. Upper red marl and gypsum. 



A detailed description of the two last of these subdivisions (which together 

 constitute what has generally been called the new red sandstone formation) 

 will not be attempted in this paper. They are only introduced for the pur- 

 pose of explaining the relations of the inferior groups. 



§ 1. Lower red Sandstone. — Pontefract Rock of Mr. Smith. — Rothe-todte- 

 liegende of the German Geologists. 



By the lower red sandstone, I mean the lowest member of the group of the 

 new red sandstone, which in Yorkshire and Durham is interposed between 

 the carboniferous order and the strata of magnesian limestone. During my 

 first visit to the county of Durham, I examined many sections which exposed 

 the junction of this limestone and the coal-measures, in the hopes of discover- 

 ing the existence of beds of conglomerate resembling those which, in the 

 neighbourhood of the south-western coal districts, are spread over the inclined 

 edges of the older formations. In this hope I was generally disappointed ; 

 but I found that the lower beds of limestone were occasionally arenaceous. 



