402 J. C. Mansel-Pleydell — Geologtj of Dorset. 



action took place, and much carbonic acid was given off. The 

 insoluble residue, consisting chiefly of silica, alumina, and a little 

 protoxide of iron, was not unusually large compared with the bulk 

 taken. The solution contained a very large amount of protoxide of 

 iron, with alumina, and a trace of peroxide of iron. Only a very 

 small trace of lime could be discovered ; nothing else was looked for. 

 Translated for the sake of clearness, this gives — 



SiOj -|- AI2O3 etc (insoluble residue). 



re..Oo a trace. 



FeO . 

 AI3O3 

 CaO . 

 CO, . 



in large quantity. 



a trace. 



in large quantity. 



It was not considered necessary to make a quantitative examina- 

 tion ; for the above could leave no doubt as to the composition of the 

 nodule. It is a clay-ironstone, made up of carbonate of iron, proto- 

 silicate of iron, and clay, or silicate of alumina. 



These specimens are therefore useful in placing before us the 

 exact method of transformation of clay -ironstone into nodular hsema- 

 tite. It is only by the infiltration of water containing oxygen and 

 other chemical agents that these could have been effected, for 

 ordinary atmospheric air could have had no access to them, some of 

 these Tertiary ironstones having been got from beds of clay 20 feet 

 thick, and often covered by from 10 feet to 30 feet of glacial drift. 

 Indeed, the effect of the atmosphere would be merely to form a thin 

 crust of brown oxide which would not be likely to consolidate, but 

 would be removed almost as soon as formed by the weather.^ 



It seems worthy of remark, however, that these Tertiary iron- 

 stones are very nearly as fully developed, both in their original and 

 metamorphio stages, as their more ancient brethren of the Carboni- 

 ferous period. And it appears, that just as we have perfectly con- 

 solidated rocks of all ages, so with the power of water to produce 

 complicated changes on all rocks through which it may be able to 

 percolate, even when they are in part composed of constituents 

 apparently so unsusceptible to its influence, or belong to a period 

 that may be called the yesterday of Geological history. 



III. — A Bbief Memoir on the Geology of Doeset. 



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



Part I. 



THE Geology of Dorset presents an almost consecutive series of 

 formations from the Liassic to the Quaternary. The Ehsetic 

 beds which have so large a development in the neighbouring counties 

 of Somerset and Gloucester, just touch the confines of the county near 

 Lyme Kegis, and were described by Mr. H.W. Bristow, F.E.S., Director 

 of the Government Geological Survey, at the Bath Meeting of the 



' The appearance of clay-ironstones which have been exposed to the weather for a 

 long period is very different from that of any of these ; and the thin crust of very 

 incoherent oxide is mure, highly hydrated, its composition being Fe4HgOg. — Manual 

 of Geology (Jukes and Geikie), pp. 15 and 23. 



