internal Structure of the Magnesian Limestone. 81 



of fragments or concretions of red and yellow ochre. The finer portions resemble many varieties 

 of the lower red sandstone ; but the coarser beds contain fragments of sandstone and water-worn 

 fragments of mountain limestone*. 4. Beds of red, grey, and yellowish marls, containing nodular 

 concretions and thin bands of limestone. The whole thickness about fifteen feet. In the lower 

 part, the clay is generally of a red tinge, and the limestone nodules partake of the same colour. 

 In the higher portion, many of the irregular bands of limestone, like the marls in which they are 

 imbedded, are of a greyish colour. Externally they are sandy, and sometimes micaceous ; but 

 the centre of the larger concretions commonly exhibits a nearly compact and pure limestone 

 containing a few obscure traces of bivalve shells. 5. Immediately over the preceding beds are 

 some thin bands of yellowish marls, surmounted by the great deposit of magnesian limestone in 

 the ordinary form in which it is developed in that district. 



At Palterton and Bolsover in Derbyshire, the sections through the corresponding deposits ex- 

 hibit the following succession, beginning as before with the lowest beds. 1. Common coal shale. 

 2. Soft light-coloured sandy shale with vegetable impressions. The two preceding are members 

 of the coal formation ; their junction with the next superior beds is unfortunately not well ex. 

 posed. 3. Beds of yellowish clay, with some carbonaceous matter apparently derived from 

 vegetable fossils. In it are thin beds of blue limestone, with bivalve shells, and, in a few instances 

 containing small fragments of carbonized wood. Many of the specimens resemble hard varieties 

 of shelly lias ; others are meagre, impure, and sandy. The thickness of this system is about six 

 feet. 4. Red and yellow clay, with beds of blue and red shelly limestone. Some of the varieties 

 of limestone are very impure ; other parts are compact or semicrystalline, and in hand specimens 

 might be mistaken for mountain limestone. The thickness of these beds is about nine feet, 

 5. Over the preceding are the lower beds of the yellow magnesian limestone. 



There is obviously a g-reat analogy between the two preceding' sections. 

 The conglomerate of Kirkby Wood-house does notj however, appear at Pal- 

 terton and Bolsover; and the beds of limestone are thicker and much better 

 developed in the latter section than they are in the former. The organic 

 remains appear to be the same in both, and consist almost exclusively of small 

 bivalve shells, which are generally too much imbedded to exhibit specific cha- 

 ractersf. To what formation shall we then refer these two deposits? The 

 red marls and conglomerate seem to connect them with the inferior red sand- 

 stone ; but the beds of limestone seem on the other hand to unite them with 

 the next superior group of marl-slate and compact limestone. To avoid all 

 ambiguity, I have placed them in a group by themselves, an arrangement 

 which can lead to no mistakes, though the subdivision may perhaps be 

 thought too unimportant to deserve so formal a notice. 



§ 3. Great middle deposit of Yellow Magnesian Limestone. 

 The deposit I am now about to notice, not only occupies the greatest part 



* This is perhaps the only instance in which a conglomerate containing pebbles of mountain lime. 

 stone is found in association with any of the groups described in this paper. (See Plate V. fig. 1.) 



t As these beds of limestone contain no magnesia, tliey are extensively quarried, and much 

 used in agriculture. 



VOL. III. — SECOND SERIES. M 



