338 Prof. A. C. Ramsay on Geological Ages 



occupied in the deposition of certain members of the Mesozoic series. 

 This may be attempted, partly on stratigraphical and partly on palseonto- 

 logical considerations. 



The Lower Lias, at its junction with the Middle Lias, or Marlstone, 

 passes gradually into that formation on the coast-cliffs of Yorkshire, 

 where it is impossible to draw a boundary-line between them, either 

 lithologically or palaeontologically. Both contain- beds of the same kind 

 of ironstone ; and the marly and somewhat sandy clays, through about 

 twenty feet of strata, are similar in character, while a good proportion 

 of the fossils in these passage-beds are common to both formations. 

 Higher up, where the Marlstone becomes more sandy, a suite of fossils, 

 to a great extent new, appears, due apparently to altered conditions of the 

 sea-bottom : the water was shallower and nearer shore ; and the topmost 

 strata of sandstone often contain many stem-like bodies, sometimes two or 

 three feet in length, lying on the surfaces of the beds in curved lines, the 

 same stems sometimes bending and crossing each other in a manner that 

 strongly reminds the observer of the broken stalks of Laminarian seaweeds 

 lying on a sandy shore, within close reach of a Laminarian zone. Taking 

 these things into account, there seems to be a much more intimate connexion 

 between the Lower and Middle than there is between the sandy beds of 

 the Middle Lias and the Upper Lias clays of Yorkshire, between which, 

 though there is a perfect conformity, yet a sudden break in lithological 

 character occurs, accompanied by a nearly complete change of fossil 

 species. But the three divisions being conformable to each other, the 

 diversities of fossil contents, more or less, seem to be owing to changes 

 in the physical condition of the sea, caused, in the case of the Upper Lias 

 shale, to sudden depression of the area, which resulted in the deposition 

 of the muddy sediments of the Upper Lias in deeper water than that 

 which received the uppermost sediments of the Marlstone. In the 

 Midland Counties, however, the lithological break between the Middle 

 and Upper Lias is not so sudden, and, in that region, there is a greater 

 community of species. 



In Yorkshire the strata immediately above the Lias are of mixed ter- 

 restrial, freshwater, and marine beds ; but even there and in the middle of 

 England, as shown by Dr. Wright, there is a certain community of fossils 

 in the passage-beds that unite the Upper Lias to the Inferior Oolite. 

 There is no perfect stratigraphical or palreontological break between them ; 

 and when we pass in succession through all the remaining members of the 

 truly marine Oolitic series of Gloucestershire, Somersetshire, and Dorset- 

 shire, no real unconformity anywhere exists. The same species of fossils, 

 in greater or less degree, are apt to be common to two or more formations ; 

 for example, such community exists between the fossils of the Inferior 

 Oolite and those of the Cornbrash, between those of the freestones of 

 the Inferior and Great Oolites, of the Stonesfield and Colly weston slates, 

 and between those of the Kimmeridgo and Portland Oolites. 



