internal Structure of the Magnesian Limestone. 101 



§ 4. Lower Red Marl and Gj/psum. 



The existence of this deposit in the higher part of the dolomitic series had 

 been known in Yorkshire many years before the appearance of any of our 

 geological maps. Mr. Smith was, however, the first to give the proper im- 

 portance to it, by determining its exact relations, by tracing its rano-e, and 

 limiting its extent. In the neighbourhood of Doncaster and Ferry BvkWe 

 there are several ancient pits from which the beds of plaster rock have at dif- 

 ferent times been extracted : and they have been found in so many wells, 

 works of drainage, and other artificial excavations, opened on the line of 

 range, that there can be little doubt of their continuity from the confines of 

 Nottinghamshire to the south bank of the Wharfe near Tadcaster. 



The bottom beds of this deposit are not visible in any section with which 1 

 am acquainted ; but they have been reached by wells sunk in the neighbour- 

 hood of Ferry Bridge, and are said to be composed of yellowish marls, making 

 a passage into the inferior beds of yellow limestone. The central beds are 

 generally composed of red and variegated unctuous marls and gypsum, not 

 distinguishable from the upper gypseous marls of the new red sandstone. The 

 highest part of the series is sometimes represented by a stiff blue clay ; but 

 perhaps more frequently by beds of red, grey, greenish, or yellowish marls, 

 rather meagre to the touch, and containing a little fibrous gypsum. 



A correct notion of the nature of this deposit will be conveyed by the fol- 

 lowing sections. 



1 . Plaster-pit Hill near Ferri/ Bridge. 



Feet. 



1. Various beds of gypseous marl resting on the yellow limestone which crops-"! 



out on the west side of the quarry. Thickness not exposed / 



2. Blue, red, and variegated marl with much fibrous gypsum lH 



3. An irregular bed of red marl, with strings and nodules of gypsum 6 



4. Hard chocolate-brown and reddish brown marl, with eight or ten thin beds"! .„ 



of fibrous gypsum / 



5. Red and blue marl beds contorted and passing into the diluvial covering 8 



The whole deposit must, in this locality, be of very unusual thickness, perhaps not less than 



one hundred feet ; for there appears to be a considerable thickness of marls, not seen in the 

 section, which are interposed between the red and blue bed (No. 5.), and the superior lime- 

 stone. There are some fissures in this quarry which contain fine crystals of transparent selenite. 



2. Section exposed hy the cut for the Canal beloio Kiiotlingley . 



Feet. Inches. 



1. Red marl and fibrous gypsum at the bottom of the section. The whole! ^ ^ 



thickness not exposed : the part visible about J 



2. Impure yellowish clay ^ ^ 



