185/.] PHILLIPS — OOLITE AND LIAS, YORKSHIRE. 97 



ft. in. 



Upper nodular band of ironstone 7 



Shale 3 



Lower nodular band of ironstone 6 



Shale 7 6 



Marlstone 1 9 



Sandy shale 20 



Farther to the south this poor representation of the great Iron- 

 stone and Marlstone series of the Yorkshire coast is traceable ; until 

 in Lincolnshire, Rutland, and Northamptonshire it resumes much of 

 its thickness and importance, though not as yet its commercial 

 value. 



Conclusion. — On reviewing the facts here selected to illustrate and 

 exemplify the variations of the middle and lower Oolites, and upper 

 and middle portions of the Lias of Yorkshire, we find sufficient 

 grounds to mark out approximately the geographical ranges of some 

 remarkable mineral conditions — and accumulations of organic re- 

 mains, — indicative of peculiarities in the depth and currents of the 

 sea, the direction and proximity of land, and other great charac- 

 teristics of the mesozoic period in this part of the earth's surface. 



Reserving for another communication the development of this 

 subject, which cannot be properly examined without additional de- 

 tails, I desire to call attention in the mean time to a few prominent 

 data. (See Section No. 6, PI. VI.) 



The whole series of strata from the Cornbrash to the Marlstone, 

 inclusive, grows thinner towards the south, and in a less degree 

 towards the west ; so that from the sea-coast near Whitby to the 

 hills near Tbirsk the thickness decreases from 1000 to 590 feet. 

 This appears in each of the main groups. Near Whitby and Guis- 

 borough the marlstone and ironstone occupy 150 feet of the cliffs ; 

 but near Thirsk only a few feet. The Upper Lias measures 200 feet 

 near Whitby, but near Thirsk less than 120 feet; and in this 

 general diminution each part partakes, — though perhaps the upper 

 part, which is the favourite seat of Nucula ovum and Ammonites 

 bifrons, has been most severely truncated. 



The great series of sandstones, shales, coal-plants, and ironstones, 

 lying above the Lias and below the Oolite of Gristhorp, 500 feet 

 thick in the Peak and the grand range of cliffs at Staintondale, is 

 only 270 feet thick near Thirsk, and is further reduced near Brands- 

 by, until on the banks of the Derwent it is scarcely traceable. The 

 upper carbonaceous series of the coast is of nearly the same thick- 

 ness at Thirsk, on a line from east to west ; but the Oxford Clay 

 diminishes from above 100 to less than 30 feet. 



Lines may be drawn in directions not deviating much from E.N.E. 

 to W.S.W. which shall coincide with bands of equal deposition, or 

 equal disappearance, of particular mineral sediments, and particular 

 distributions of organic life. For these lines — as defining equality of 

 earthy sediments, — and thus sometimes marking out similarity of 

 sea-depth and current-action — the author proposes the term " iso- 

 chthonal," and is of opinion that the tracing of them will hereafter 



VOL. XIV. PART I. H 





