302 R. Cuoksey Burton — Yellow Sands and Marl. 



Magnesian Limestone. In the present instance, however, the Marl 

 Slate is absent for reasons given in detail below. At Frenchman's 

 Bay the ' Yellow Sands ' outcrop about 20 feet below the level 

 of the cliff foot, and at Trow llocks the above-mentioned included 

 beds of ' Yellow Sands ' occur at various heights (4-30 feet) above 

 the base of the cliff, so that these beds are sometimes as much as 

 50 feet above their natural position. The following are possible 

 explanations of their mode of formation : — 



1. The sand may be the residue left on the solution of the 

 Magnesian Limestone. 



2. The sand may have been carried up from beneath the Marl 

 Slate by the thrusting movements. 1 



3. The beds may be a later deposit of ' Yellow Sands ' laid down 

 in the positions in which we now find them. 



4. They may have been carried up from below by the action of 

 underground water. 



We will deal with these suggestions in order. 



1. While studying the mineralogical composition of the 'Yellow 

 Sands ' we have examined many samples of Magnesian Limestone, 

 and in some cases they have been proved to contain quartz grains and 

 various heavy minerals, which we hope to describe at some future 

 date. These Magnesian Limestone grits occur at Rocky Crag, Tunstall 

 Hill, in the form of a vertical dyke-like mass, at Marsden Quarry as 

 a horizontal bed in the Middle Magnesian Limestone, and in a cave in 

 Marsden Quarry about 40 feet above the level of the last. In all 

 these cases, however, the residue left from the limestone after 

 treatment with acid consists of angular and rounded grains of various 

 sizes from -^ inch to microscopic proportions ; the angular grains are 

 among the largest, and consist of quartz, chalcedony, and chert. This 

 sand is evidently not wind-blown but deposited by water, and could 

 not be mistaken for the ' Yellow Sands ', since in the latter the 

 lai'gest grains are perfectly rounded and abraded ; the smaller grains 

 of the ' Yellow Sands ' are sometimes quite angular. This, however, 

 is in accordance with Sorby's statement that the smallest grains in 

 a sand are always the most angular, as they present only a small 

 surface for abrasion. It is thus evident that the beds of ' Yellow 

 Sands ' are not insoluble residues left on the solution of the 

 Magnesian Limestone. As regards the origin of the chert in these 

 sandy residues, it may be stated that we have lately discovered in the 

 brecciated Magnesian Limestone, nodules, irregular beds, and brecciated 

 pieces of chert. On the coast half a mile north of Eyhope a thin 

 brecciated bed of white chert occurs 30 feet long and 6 inches thick ; 

 it is irregular and forms part of the breccia, although not so strongly 

 brecciated as the limestone owing to its being of stronger material. 



1 The effects of the horizontal thrust on the Permian of this area have been 

 fully described by Dr. Woolacott in Memoir No. 1, Durham University Philo- 

 sophical Society. For our purposes it is sufficient to state that considerable 

 horizontal movement of the upper beds of limestone has taken place over the 

 lower, causing brecciation of the lower and middle Magnesian Limestone ; part 

 of the lower limestone lying under the thrust-plane is slightly disturbed, but 

 remains unbrecciated. 





