G. A. Lebour — Limits, of the Yoredale Rocks. 539 



Plate XIV. 

 Fig. 1. — Orbitoides papyracea (Boubee). 



a. Lateral aspect. Magnified 5 diameters. 



b. Periphero-lateral aspect. Ditto. 



c. Horizontal median section. Magnified 20 diameters. 



d. Central transverse section. Ditto. 

 Fig. 2. — Orbitoides dispansa (Sowerby). 



a. Lateral aspect. Magnified 6 diameters. 



b. Periphero-lateral aspect. Ditto. 



c. Central transverse section. Magnified 20 diameters. 

 Fig. 3. — Orbitoides Sumatremis, sp. nov. 



a. Lateral aspect. Magnified 6 diameters. 



b. Periphero-lateral aspect. Ditto. 



c . Horizontal section, near the median plane. Magnified 35 diameters. 



III. — On the Limits of the Yoredale Series in the North of 



England. 1 

 By G. A. Leeouk, F.G.S. London and Belgium, F.R.G.S., etc. 



Lecturer on Geological Surveying at the University of Durham College of 

 Physical Science, Xewcastle-on-Tyne. 



WHEN a group of beds has ■well-defined boundaries above and 

 below, and when moreover its palaeontological characteristics 

 coincide with its stratigraphical limits, it becomes a boon alike to the 

 field-geologist and to the fossil collector. When, on the other hand, 

 both limits and fossils fail to enable one to follow a group beyond 

 a certain point, the sooner the seiies as. such is relegated to the limbo 

 of purely-local, convenient, but untrue divisions, the better. I pro- 

 pose to show in this paper that the important group of beds com- 

 monly known as the Yoredale Series in the North- Western parts of 

 Yorkshire is a group of the latter kind, convenient indeed in that 

 district, but quite incapable of being traced much further North 

 either stratigraphically or palaeontologically. 



I cannot find a more concise definition of what is usually meant 

 by the term "Yoredale Series" than that given by my friend 

 Prof. Nicholson. 2 He says : " The Yoredale Series of Phillips, the 

 Upper Limestone or Limestone- shale series of some authors, con- 

 sists of numerous alternating beds of limestone, sandstone, grit, 

 and shale, with a few thin and worthless seams of coal, the whole 

 attaining a thickness of 500 feet, according to Mr. Eorster. The 

 two most constant members of the Yoredale Series are the 'Tyne- 

 bottom Limestone,' and the 'Main,' 'Great,' or 'Twelve-fathom' 

 Limestone, respectively the lowest and the highest limestones of the 

 group. As regards Cumberland and Westmorland, the Yoredale 

 Series is best studied in Alston Moor, in Teesdale, and along the 

 .summit of the Pennine Escarpment ; but for its fullest development 

 we must look to the valleys and hills of Yorkshire, where it was 

 originally described by Prof. Phillips, and where it sometimes 

 attains a thickness of 1000 feet." Now this set of beds is bounded 

 above by the Millstone-grit, and below by the Scar Limestone 



1 Bead at the Bristol Meeting of the British Association, 1st September, 1875. 



2 Essay on the Geology of Cumberland and Westmorland, by H. A. Nicholson, 

 .D.Sc, F.G.S. , etc., London and Manchester, 1868, p. 79. 



