438 J. C. Mansel-Pleydell — Geology of Dorset. 



thickness of the basalt at only 3000 feet, the pressure on each square 

 yard of underlying chalk would be about 2000 tons.^ 



Tlie analysis, which is extremely similar to one by Mr. Wonfor, of 

 the Chalk of Cushendall, Co. Antrim,'* shows that it is a limestone of 

 very great purity, the per-eentage of siliceous matter being so small 

 as to be quite insignificant. It should therefore be of the highest 

 value in many chemical manufactures, especially that of bleaching 

 powder. But it is remarkable that although in the North of Ireland 

 an immense quantity of this material is used up, it is not made 

 there, but is mostly imported from Glasgow and Lancashire. So far 

 as I know, there is not a single Ohloride of Lime Works in Ulster.^ 



III. — A Brief Memoir oe the Geology of Dokset. 



By J. C. Mansel-Pleydell, F.G.S. 

 Part 11. 

 {'Continued from page 413.) 



AS the sands and clays of the Hastings series lie conformably on 

 the Purbeck Beds, it is probable that the same area which formed 

 the embouchure of the Purbeck river performed still the same office 

 during the Hastings Sand age ; but the entirely different character of 

 the deposit shows at least that the soil of the country drained by the 

 latter was different from that which supplied the former ; and it is 

 evident also that, in the district under, consideration, the motion of 

 the water of the Hastings river was much more rapid, from the 

 abundance of sand, coarse quartz, and gravel with pebbles. 



This lower member of the Wealden consists of sand, sandstone, 

 calcareous grit, and shale. At Swanage and Worbarrow the cal- 

 careous grit alternates with red and green sandy clay ; it contains 

 bones of the Iguanodon and portions of silicified coniferous trees, 

 the stone into which they are converted being dark -brown in colour, 

 and receiving a fine polish. It does not effervesce with acid. The 

 Hastings Beds form the north side of Swanage and Worbarrow 

 Valley, and pass through Godlingston, Corfe, Church Knoll, Steeple. 

 and Tyneham ; a small patch appears at Mewps Bay, Lul worth, and 

 Man of War Cove. Their junction with the Purbeck Beds is favour- 

 ably exposed at Worbarrow. In the little cove between the Tout 

 and Gad Cliff, about fifty feet of clay alternates with beds of con- 

 torted limestone ; at Swanage it is invisible, being masked by a 

 fault. The only other appearance of this bed occurs between Chaldon 

 and Holworth, flanked by the Greensand on the south, and the 

 Eidgeway fault on its northern side. It is evident from our review 

 of the Wealden and Upper Oolite Beds, as represented in this neigh- 

 bourhood, that they are quite unconformable to the Cretaceous system, 

 which not only overlaps them gradually, but covers them occa- 



^ The Chalk of Tyrone is in fact curiously shattered and split up into small irregular 

 parallelepipeds, which appears to be due to more than ordinary jointing. The great 

 pressure may have had something to do with it. 



2 Journ. Royal Dub- See, July, 1860. 



^ A quantity of the Antrim Chalk is, however, exported to England for manufac- 

 ture there. 



