Revieivs — 8trahan's Model of the I, of Furheclc. 275 



VII. — Memoirs of the Geological Survey of England anb Wales. 

 Guide to the Geological Model of the Isle of Purbeck. 

 By Aubrey Strahan, M.A., F.E.S. 8vo ; pp. 26. (London, 

 1906 : sold by Edw. Stanford, 12-14, Long Acre. Price 

 sixpence.) 



EAELY in the present year an interesting and instructive addition 

 was made to the collection of the Museum of Practical Geology 

 in Jermyn Street. This was a geological model, on a horizontal scale 

 of six inches to a mile, of the Isle of Purbeck, prepared by Mr. J. B. 

 Jordan, under the supervision of Mr. Strahan. This model is in 

 three separate pieces, and, in order to give the physical features 

 due prominence, the vertical scale has been made twice as great 

 as the horizontal. Sections are drawn on the edges of the three 

 blocks of the model, and these sections are carried down to 660 feet 

 below sea-level. By this means the interior geological structure 

 as well as the outcrop features are brought to the notice of the 

 observer. 



In further illustration of this excellent piece of work a Guide 

 to the Geological Model, compiled by Mr. Strahan, has recently 

 been issued by the Geological Survey. This " Guide " is 

 furnished with two plates, one being a photograph of the model, 

 and the other a small but very clear geological map of the Isle of 

 Purbeck, on the same scale as the photograph, each fitting lengthwise 

 into a full-sized octavo pamphlet. In addition to these illustrations 

 there is a page giving the sections which are drawn on the edges 

 of the three blocks of the model, as shown on the photograph of 

 the model (plate i). 



There can be little doubt that the Isle of Purbeck and its 

 natural prolongation along the Lulworth coast present us with 

 one of the most interesting, and at the same time accessible, pieces 

 of geological structure to be found anywhere. Omitting the purely 

 Bagshot country in the northern part of the ' Isle,' where the 

 strata are flat and much obscured by peat, etc., we have in the 

 southern half of the Isle of Purbeck the features of a varied and, 

 in some cases, complex stratigraphy presented to the observer. 

 Indeed, the region from Gadcliff almost to Whitenothe is a mountain 

 chain in miniature dissected by the sea. 



Mr. Strahan has not been slow to point out the leading features 

 in the geology of the district — the long and lofty Chalk ridge, the 

 Wealden valley, where the softer beds have yielded more readily 

 to erosion, and then again the resistance offered by the harder 

 Purbeck and Portland limestones. " The scenic features due," he 

 says, " to this sequence of hard and soft beds are reproduced 

 wherever the strata come to the surface, no less in the narrow 

 outcrops near Durdle and Lulworth than in the broader tracts near 

 Swanage." The different results of subaerial and marine erosion 

 are indicated. In the sea-coves may be found the most perfect 

 illustration of the action of the sea upon inclined strata of unequal 

 hardness. 



