PORTLAND ROCKS OP ENGLAND. 221 



they must have heem deposited during the period in which, at Tis- 

 bury and Portland, the remainder of the flinty series, the Whit-bed 

 and the Eoach were being formed, and they are thus older than some 

 portions of the Portland rocks. 



Summary. 



The facts arrived at by this investigation may be briefly recapitu- 

 lated as follows : — 



In all the sections near the coast, the Purbeck is separated from 

 the Portland by a line of clay ; but the uppermost bed of the former 

 is not always the same, and the line of junction, though not eroded, 

 is irregular. The Portland series shows first the "Whit-bed and 

 Eoach characterized by particular fossils, and esi^eciaXlj hj Ammonites 

 giyantevs, and lying with local unconformity on the next beds. 

 The characters of this part differ in the various localities ; and it 

 almost thins out at Upway. It may be known as the Building-stone 

 series. Below is the Flinty series, divisible into several parts, highly 

 fossiliferous at the base, and characterized by Ammonites holoniensis 

 &c. This isthickest at St. Alban's, and becomes very chalky at Upway. 

 The Portland Sands contain a variety of beds (clays, cement-stones, 

 and oyster-beds), and have a peculiar fauna distinct from the lime- 

 stones above ; but these characters are not constant. The thickness 

 must be assumed much greater than has usually been done, unless 

 the limit of the Kimmeridge Clay is unduly raised, and it is esti- 

 mated at 277 feet. The Boulognian episode to which, unjustifiably, 

 the name of Lower Portland has been given, is represented by normal 

 shales and cement- stones on the Kimmeridge coast, aU the beds 

 being here much expanded, but recognizable by their general suc- 

 cession and the introduction at definite horizons of the character- 

 istic invariant fossils. 



In the Yale of Wardour the Purbeck is also marked off from the 

 Portland by a band of cla}^, the succession is very similar, and beds 

 corresponding to the Whit-bed and Roach may be recognized. The 

 Plinty series is far less flinty and more chalky, and it has at its base 

 a well-marked zone of fossils. A development of it downwards 

 takes place here in the shape of some sandy freestones largely 

 worked, containing several layers of Tricjonioe^ especially one at its 

 base. Below the limestones, the true Portland Sand is very thin 

 and brown, and is underlain by a curious rubbly kind of stone ap- 

 pearing to belong, by its fossils, to an earlier date. 



At Swindon the relations of the Purbeck to the Portland are 

 most remarkable, the latter being carved out in hollows which contain 

 rolled blocks of it, evidencing a land surface and rapid changes ; but 

 as the uppermost part of the Portland here corresponds only to the 

 top of the freestones of Tisbury, and the higher parts are wanting, 

 this erosion may have taken place in Portland times. The main mass 

 of the quarried stone is of the same ago as that at Tisbury ; and at 

 the base we find some fossiliferous beds in various well-marked blocks, 

 and containing a peculiar fauna ; these 3^n^om'a-beds have a conglo- 



