

STRATIFICATION. 71 



On Stratification in general. 



Strata, the term defined. Strata, layers, and beds are synonymous 

 terms. "Strata," says Professor Playfair, "can only be formed by- 

 seams which are parallel throughout the entire mass." This defini- 

 tion was founded upon the supposition that loose materials deposited 

 under water must be arranged in layers parallel to the surface of 

 the water ; it undoubtedly contains the general or fundamental 

 idea of stratification, but is often too abstract for practice. It 

 includes too much, for slaty cleavage produces truly parallel laminae ; 

 and it excludes many layers produced under greatly agitated water, 

 on lines of sea-coast, and in the direction of sea-currents, such as 

 conglomerates, false bedding, &c. The most remarkably regular and 

 parallel seams or divisions between strata happen in calcareous and 

 argillaceous rocks ; but the partings in sandstone are much less 

 uniform. A particular shelly bed of stone lies at the top of the 

 coralline oolite of Yorkshire, and may be traced for a great dis- 

 tance ; a red rock, long since noticed by Lister, lies at the base of 

 the chalk of Yorkshire and Lincolnshire, and cut through by the 

 Wash, reappears in the same position in Norfolk, for sixty miles 

 in compass ; the cornbrash limestone, seldom more thaii ten feet in 

 thickness, is continuous from Dorsetshire nearly to the Humber, 

 and reappears in the cliffs at Scarborough. ' In these instances, 

 therefore, Playfair's definition applies very well. On the contrary, 

 the beds of sandstone with coal which are interposed in the Lower 

 Oolite system of Yorkshire, are altogether five hundred feet thick near 

 Robin Hood's Bay, but dwindle toward the south, and are entirely 

 deficient before reaching the Derwent 



Such beds are therefore wedge-shaped ; and cases sometimes occur, 

 as in the Lincolnshire limestone, where, by attenuation in all direc- 

 tions from the centre, they become lenticular. See fig. 25 for these 

 and other appearances. 



Interposed Strata. The strata, therefore, are not all co-extensive. 

 Limestones and thick clays are probably the most persistent and 

 regular, sandstones the most limited and local. Local modifications 

 or interposed beds, due to conditions of the sea-bed, cause the principal 

 differences between distant portions of the same formation. 



The Lias of England rests immediately upon red and bluish 

 marly clays with white gypsum ; at Luxembourg these strata are 



separated by a thick sandstone. 



In the north of England, Mag- 

 nesian Limestone separates the 

 Coal Measures from the New Red 

 Sandstone; but in other parts of 

 the island these two formations are 

 in contact. In the breast of Ingle- 

 borough, the limestone beds are 

 aggregated into one vast mural 

 precipice or scar ; but as we proceed northwards, this mass opens and 



