NA TURE 



45 7 



THE FRILLED FRINGE OF THE SOUTH 



COAST. 



The Geology of the Isle of Purbeck and Weymouth. 



By A. Strahan. Pp. xi + 278. (London: Printed 



for her Majesty's Stationery (Jlifice, by Wyman and 



Sons, Ltd., 1898.) 



ANYONE travelling from London to the South Coast 

 may see how the gentle southerly inclination of 

 the chalk, which has carried it tmder the Tertiary beds, 

 changes on the south side of the trough, bringing the 

 chalk up again with a steeper slope, but now with a 

 northerly dip ; how it then folds over the a-^is of the 

 Wealden anticlinal, only to fall to the south more rapidly 

 than it did before, and pass under the waters of the 

 Channel. If we make oar traverse further west, there, in 

 consequence of the strike of the rocks being oblique to the 

 trend of the coast, we find still more southerly folds 

 brought into view, and the chalk, after passmg rapidly 

 under the Tertiary beds of the Hampshire basin, re- 

 appearing in the Isle of Wight, with the strata vertical 

 or even thrown over beyond the vertical. Along this 

 line of disturbance older and older Mesozoic rocks turn 

 up in the anticlinal folds. 



Here, as in many other parts of the world, we have 

 plications increasing in sharpness as we cross the strike 

 towards some axis of principal intensity. As might be 

 expected, these folds and their accompanying disruptions, 

 being of the nature of local readjustments in an area of 

 long-continued crush, are seen to be of different dates. 

 When we carry our investigations further afield we find 

 that the disturbances of the strata, which in this district 

 are shown to be modern and probably still going on, 

 belong to a very ancient system of strains, for the 

 Paleozoic rocks of Belgium, Dover and Somerset are 

 still more severely contorted, and the Mesozoic form- 

 ations! rest almost undisturbed on the upturned and 

 eroded edges of these older previously folded beds. 



The author of the work before us has found himself 

 called upon to describe and explain the structures and 

 sequence of rocks occurring in one of the most interest- 

 ing of these crumpled coast districts. His reputation as 

 one of the soundest of the younger school of geologists, 

 as well as the names of those of his colleagues who have 

 contributed to the work, are a sufficient guarantee that 

 it is an important addition to scientific literature. 



The public hardly realises what an immense amount 

 of valuable scientific work is contained in the maps and 

 memoirs of the Geological Survey, and the accuracy of 

 even the older Survey work is marvellous if we take into 

 account how much of it may be considered as the efforts 

 of pioneers, carried out when comparatively little was 

 known of the methods of discrimination and the data 

 for classification which are now available. 



A work of such importance as this by authors of such 

 position in science as those who have contributed to it 

 would have been very differently turned out by the far- 

 sighted American Government, by artistic Italy, or 

 spirited little Portugal. To spend so much on a scien- 

 tific staff, and then depreciate the results in the face of 

 NO. 1533, VOL. 59] 



the world by issuing them in the unattractive form in 

 which our Survey Memoirs are published, is not business- 

 like, to say The least of it. That the present volume is 

 above the average is clearly due to careful drawing and 

 revision by the author rather than to an improvement in 

 the system, which we see in the relegation of the valuable 

 ;-A//;«t' by the Director-General to such small print as 

 would seem to indicate as clearly as if it were printed at 

 the head, " Preface, of no importance, pass on to the 

 next," or which gives in some of the sections a lot of 

 small blots and blobs, often unreadable with a lens, to 

 represent the numerals referred to in the text ; while the 

 ink, which is superfluous here, might have improved the 

 capitals elsewhere. 



The work is a treatise upon the geology of a defined 

 area, and to give an account of this would be to make 

 an abstract of a book already too much condensed. The 

 formations included range from the Fullers' Earth to 

 Recent, but the greater part is devoted to the Jurassic 

 and Cretaceous Series ; the Tertiary and Recent beds 

 do not occupy many pages. The rocks are much dis- 

 turbed, and an interesting account is given of the 

 character and age of the earth movements which have 

 affected them. No molten matter has burst through 

 these broken strata. The crush has been in some cases 

 so severe that the flints have been reduced to powder, 

 and drawn out into black streaks like so much coal dust 

 (p. 179), while large masses of rock have been thrust 

 forward, the older being often pushed over the newer in 

 vast slices, and portions pinched out so that the thickness 

 has been reduced to one-fourth of what it was. The 

 structure of the district is complicated by crossing systems 

 of folds which produce oval or spoon-shaped basins and 

 domes, varying according to the relative intensity of 

 either system. As these are arranged in quincunx 

 pattern they frequently come into view eti echelon. The 

 classification and nomenclature have demanded the con- 

 sideration of some theoretical questions of scientific 

 importance ; and here especially we have to thank the 

 officers of the Survey for using words which convey a 

 clear idea to the public of what they are talking about, 

 instead of endeavouring to show their own ingenuity, 

 wide reading or knowledge of foreign languages in the 

 invention or adoption of new and unnecessary terms. 



If there is a doubt as to the horizon at which the 

 boundary between the Jurassic and Cretaceous systems 

 should be drawn — and a suspicion arises that the 

 equivalent position has not been assigned to it in different 

 areas — we must inquire whether this difficulty arises from 

 wrong identifications among the stratified rocks in either 

 of the localities, or whether there has not been some 

 change of conditions which has made it difficult to prove 

 e.xact chronological identity between the series seen in 

 the several more or less widely separated areas. For 

 instance, land surfaces are suffering denudation, while 

 sediment is being deposited in an adjoining area in sea 

 or estuary or lake or river. In the district described 

 there are several successive deposits which have a 

 tendency to become coarser as we follow them westward, 

 or point to the local incoming of estuarine or fluviatile 

 conditions. In such an unstable area palreontological 

 continuity or interruption will depend upon slight 

 geographical changes. 



X 



