234 



NA TURE 



[January 3, 1901 



freshwater Wealden must represent true marine Lower 

 Cretaceous beds elsewhere, and that consequently it is 

 equally erroneous to classify the Wealden series entirely 

 with the Jurassic system or entirely with the Cretaceous. 

 If the planes of division between our formations are 

 more often than not ill-defined, so also are those between 

 the main systems. Between our Palccozoic and Mesozoic 

 strata there has never been a very well-marked boundary, 

 for some authorities have placed the Permian with the 

 older division and some with the newer. 



The tendency of recent investigations in the midland 

 areas is to show that a considerable series of red beds 

 which have been regarded as Permian are truly portions 

 of the Coal-measures, while it is evident that the Mag- 

 nesian Limestone series is stratigraphically united more 

 closely with the Triassic strata. In Britain the main 

 mass of the Permian (Magnesian Limestone series, &c.) 

 lies unquestionably with great discordance on various 

 subdivisions of the denuded Carboniferous and Devonian 

 rocks. Abroad in many areas, in India, Australia and 

 elsewhere, there appear connecting links in strata grouped 

 as Permo-Carboniferous ; but it is a question whether 

 the original Permian is anything more than a provincial 

 set of strata, unentitled to rank with a system (see C. R. 

 Keyes, /(?«r«. Geol.^ Chicago, vol. vii. p. 337, 1899). 



As the history of the successive strata in different 

 countries becomes better understood, so it becomes 

 possible more closely to parallel the life-epochs which 

 are represented in the rocks. Such life-epochs do not of 

 course correspond with the sedimentary changes which 

 are recorded by the rocks themselves, and hence a 

 double system of grouping becomes needful. In our 

 own country this has been long apparent, and the 

 successive groups of strata which are so well estab- 

 lished in the Ordovician and Silurian systems of Wales, 

 the Lake District and the Southern Uplands of Scotland 

 require distinct stratigraphical terms, while the life- 

 history and the correlation of the subdivisions are 

 indicated by the zonal groupings based on zoological 

 evidence. The representation on maps and in sections 

 of the main stratigraphical groups, or geological forma- 

 tions, is essential in order to show the physical structure 

 of a country, not only in reference to economic questions, 

 but also in regard to the influence on the present scenery 

 of the irocks and the movements to which they have 

 been subjected. Different types of landscape and the 

 evolution of river systems are engaging a good deal 

 of attention, notably in the United States ; and the study 

 has led to the introduction of a large number of terms 

 which are rather difficult to remember, but the more 

 important are explained in Mr. J. E. Marr's "Scientific 

 Study of Scenery." 



Increasing attention is given to the great movements 

 which have affected the rocks especially in mountain 

 regions. The pioneering work of Heim in Alpine regions 

 has been utilised and developed in the most brilliant 

 manner by Lapworth and Rothpletz and many others. 

 The old ideas of reversed faults have been, so to speak, 

 magnified into great earth movements, whereby huge 

 masses of country have been overfolded, fractured, and 

 overthrust, the older being pushed over the newer. On a 

 small scale such overthrusts were long ago recognised in 

 some coal-fields by the name of overlap faults, and the 

 displacement was measured by yards— now it is some- 

 times reckoned by miles. Moreover, not only in High- 

 land regions where the secret inferred by Nicol was un- 

 ravelled by Lapworth have these mighty overthrusts been 

 made manife.st, but on a comparatively small but by no 

 means unimportant scale they have been traced out and 

 pictured in the Cretaceous rocks of the Isle of Purbeck 

 by Mr. A. Strahan. The same observer has drawn 

 attention to other overthrusts in the great Coal-field of 

 South Wales. 



All sorts of complicated structures due to cross-folding 



NO. 1627, VOL. 63] 



and faulting, to successive horizontal displacements and 

 twisting, have been produced in mountain regions; and 

 Dr. Ogilvie Gordon has dealt exhaustively with the sub- 

 ject in a paper on the torsion-structure of the Dolomites 

 {Quart. Journ. Geol. Soc, vol. Iv. p. 560). To quote one 

 sentence from her paper will, perhaps, be enough to give 

 an idea of the puzzles she has attempted to solve : "Anti- 

 clines have been twisted round synclines, and the rocks 

 in the synclines have themselves been twisted and dis- ; 

 torted, buckled up and depressed, overthrust and faulted 

 normally, cross-faulted and cleaved, to an extent that has 

 not hitherto been realised." We may add that the sub- " 

 ject of torsion- structure has been examined mathematically 

 by Mr. J. Buchanan {Phil. Mag.., vol. 1. p. 261). 



The evidence of great folds and flexures, accompanied 

 by overthrust faults, has lately been brought more fully to 

 light in the Malvern region by Prof. T. T. Groom, while 

 in the Lake District the field labours of Mr. J. E. Marr 

 and Mr. A. Harker, as recently expounded (Proc. Geol. 

 Assoc, vol. xvi. August 1900), indicate that the country 

 has there been affected, not only by overthrust faulting, 

 but by more or less horizontal displacement, termed 

 " lag " faults, whereby lower and older strata have been 

 moved more rapidly than newer overlying strata, which 

 consequently have lagged behind. Other faults, called 

 " tears," are described, where, during these movements, 

 rents have occurred in the shifting masses of strata 

 without occasioning much vertical displacement. 



In very many cases along fault-planes there has been 

 produced a kind of breccia due to the effects of displace- 

 ment, but more striking results of such action have 

 lately been made known in the production of con- 

 glomerates. In such cases the effects of earth-move- 

 ments have not only fractured, but actually worn away, 

 the edges of the shattered rocks. In the Isle of Man, 

 where the Manx slates have undergone acute folding 

 followed by intense shearing, the shear-cleavage has cut 

 and displaced bands of grit and has actually rounded 

 the fragments so as to produce what Mr. Lamplugh has 

 termed a crush-conglomerate. His observations have 

 borne good fruit elsewhere. Mr. C. A. Matley has 

 described crush-breccias and crush-conglomerates in 

 Anglesey, where they occur "along zones of powerful 

 crushing, especially in areas where the soft, fine-grained, 

 slaty rocks alternate with tougher and more brittle 

 strata, such as grits and quartzites" {Quart. Journ. Geol. 

 Soc, vol. Iv. p. 657), and Prof Groom has dealt with the 

 crush -breccias of the Malvern Range {I'h'd., p. 151). 



It has long been felt that some revolution in palaeonto- 

 logical nomenclature is needed, and, fortunately, the 

 matter has been taken up boldly and effectively by 

 Dr. Arthur W. Rowe.^ In old times new species were 

 named from fossils obtained from formations without 

 reference to their special horizons. Some were founded 

 on the evidence of but one or two specimens, and it has 

 not unfrequently happened that " varieties " have been 

 found which preceded in time the type species. Of late 

 years, when increasing attention has been given to 

 careful collecting, there has been a tendency to " make 

 every prominent form a species, on the plea that every 

 minute variation must be ticketed and pigeon holed." In 

 this way very many of the old land-marks have been 

 removed, the study and identification of species have 

 passed beyond the comprehension of any but the spe- 

 cialist, and the value of his labours to others has been 

 more and more reduced or obscured. Dr. Rowe has now 

 for some years devoted the leisure of a busy life to a 

 careful collection of Micrasters from successive stages 

 or zones in the Middle and Upper Chalk. He finds 

 that by examining the faeies of the genus in each 

 horizon, passage-forms prove to be the rule, while 

 sharply-defined and typical species are the exception. 

 He has been able to trace an unbroken continuity in 



1 Quart. Journ. Geol. Soc, vol. Iv. p. 494. 



