496 



NA TURE 



[March 2.2, 1894 



opinions have received support from our recent surveys. The 

 upper part of the Gault becomes more sandy to the west, and 

 was there mapped as Upper Greensand ; the clay coloured as 

 G?.u]t in Wiltshire representing only about the lower third part 

 of >he Gault of Folkestone. This clay becomes so thin to the 

 west that it cannot be separately mapped. 



Mr. Jukes-Browne makes three divisions of the Gault and 

 Upper Greensand series, which are now found to constitute 

 really ore formation : — 



3. Greensands and Sandstone, and chert beds (Zone of Pecten 

 asper). 



2 Buff Sands, Malmstones, and silty Marls ; the last repre- 

 senting the Upper Gault (Zone of A7nmonites rostratus). 



I. Lower Gault Clays (Zone of Ammonites lautus and Ajnm. 

 inlerruptus). 



The Chert-beds of Wiltshire and Devonshire are local deve- 

 lopments in the Zone of Pecten asper. They are not found in 

 Dorset, but they attain importance in the Isle of Wight, and 

 were there separately mapped by Mr. Strahan. 



In the neighbourhood of Devizes the subdivisions of the 

 Upper Greensand are well marked. The lower one, or " Malm- 

 stone," contains, especially in the lower part, colloid silica in 

 the form of small round globules and sponge spicules, sometimes 

 to the extent of from 40 to 50 per cent, of the stone. The 

 upper division, about 70 feet thick, near Devizes, consists of green 

 and grey sands. As these are irregular in thickness, thin out 

 rapidly to the north, and extend as a band in a nearly east and 

 west direction, they may represent an ancient sand-bank. The 

 persistence of the Malmstone over a very wide extent of the 

 " Upper Greensand " of England is a noteworthy fact. 



A revived industry of some interest on the borders of 

 Bedfordshire and Buckinghamshire is the extraction of fuller's 

 earth from the Lower Greensand. This deposit is now 

 worked by mines on the flanks of the escarpment. Mr. 

 Cameron has frequently visited these mines, and has described 

 them in papers read before the British Association and else- 

 where. 



Jurassic. — Some of the most important recent additions to 

 our knowledge of the structure of the Jurassic and Cretaceous 

 rocks of the South of England have been made by Mr. Strahan 

 in his re- examination of Dorsetshire for the Drift Survey. The 

 area known as the Isle of Purbeck has long had a peculiar geolo- 

 gical interest, not only from the fact that the Portland and Purbeck 

 rocks there reach their maximum development, but also from its 

 structure. It is traversed by an extremely sharp and faulted 

 monoclinal fold, a continuation of the Isle of Wight monocline, 

 from which, however, it differs in being accompanied by inversion 

 of the strata and much overthrust faulting. This structure may 

 in fact be regarded as an intermediate stage between a simple 

 monocline and a complete overthrust. The deeply indented 

 coast affords unusual facilities for examining the effect of the 

 moveiuent. The old one-inch map, on account of the smallness 

 of the scale, gave merely a diagrammatic view of the structure 

 of the "island'" In the resurvey on the six-inch scale both 

 the faults and the subdivisions of the strata have been traced 

 with a detail that was before impossible. In the Isle of 

 Purbeck the principal additions to the map consist in the 

 tracing of the subdivisions of the Cretaceous system. The 

 Lower Greensand, which is so well developed in the Isle of 

 Wight, was known to exist in the Isle of Purbeck also, but its 

 limits had never been determined. It has now been separated 

 from the Wealden group, with which it was formerly confused, 

 and it has been traced westward until it finally thins away, while 

 at the same time the W^ealden Shales, which form the upper- 

 most subdivision of the Wealden group in the Isle of Wight, 

 have been traced through the Isle of Purbeck as far westward 

 as they extend. 



During the mapping of the Lower Greensand some interesting 

 evidence as to its relation with the overlying Gault came to 

 light. This evidence tends to confirm the conclusions formed 

 dunng the re-mapping of the Isle of W'ight, for the break at the 

 base of the Gault, which was there only suspected, becomes so 

 much more pronounced westwards as to suggest that the base of 

 the Cretaceous system might have been more suitably drawn 

 at the bottom of the Gault than at the bottom of the Wealden 

 group, which is inseparably connected with the Purbeck beds. 

 Moreover, a conglomerate which forms the base of the Gault 

 seems to correspond to the Carstone of the Isle of Wight, 

 which has again been correlated with the Folkestone beds. 

 The suggestion, therefore, made long ago, that a portion of the 



NO. 1273, VOL. 49] 



Folkestone beds should be included in the Upper Cretaceous 

 group receives support. In the W^eymouth Peninsula the 

 principal alterations relate to the mapping of the subdivisions 

 of the Chalk as far westward as they are recognisable, and in 

 the tracing of certain subdivisions of the Corallian rocks which 

 are locally developed near W^eymouth. The numerous faults 

 of the area have also been followed, with a minuteness of 

 detail which was impossible on the old one-inch map. An 

 interesting result has been obtained from this work. The faults 

 and foldings of the strata, though nearly all agreeing in direc- 

 tion, were found to have been formed at two different periods, 

 the one set affecting the Oolitic rocks but passing under the 

 Upper Cretaceous strata without disturbing them, the other 

 breaking through both Oolitic and Cretaceous rocks alike. The 

 older movements took place between the deposition of the Upper 

 and Lower Cretaceous strata, while the later set were obviously 

 contemporaneous with the Isle of Wight and Isle of Purbeck 

 monoclines, which are believed to be of Miocene age. In 

 more than one case, faults of the later age cross obliquely the 

 older lines of fracture, producing a complication which could 

 only be worked out on the large scale map. The break at the 

 base of the Gault mentioned above seems to have been due to 

 the faulting and upheaving of the rocks during the first of 

 these periods of disturbance. It becomes here a most pro- 

 nounced unconformability, and the Gault with a thin conglom- 

 erate at its base passes over the edges of the Wealden, Purbeck 

 and Kimeridgian rocks in rapid succession. 



Triassic. — Advantage has been taken of the prosecution of 

 the Drift Survey across the salt districts of Cheshire and 

 Staffordshire to obtain much additional information regarding 

 the Triassic rocks, especially with reference to their industrial 

 aspects. Mr. C. E. de Ranee has collected 208 sections of the 

 salt deposits at Northwich, Middlewich, Winsford and Lawton. 

 He has likewise reduced some mining plans of salt-workings 

 and placed their details on the six-inch maps, and has further 

 collected tables of the levels of the brines at various periods, 

 reducing these levels to Ordnance datum, and thus showing the 

 height of the Upper and Lower rock-salt surfaces. 



Carboniferous. — It is in the re-examination of the great coal- 

 field of South Wales that the chief recent operations of the 

 Survey in the Carboniferous system have lain. Sufficient 

 progress has now been made to show of how much practical 

 value a detailed survey of this coal-field will prove to be. Mr. 

 Strahan, who has had charge of this work, soon ascertained 

 that while the great thickness and uniformity of character of 

 the widespread "Pennant Gtit" makes it difficult to obtain 

 indications of the geological structure over large tracts of 

 ground, the position of a certain coal-seam known as the 

 " Mynyddislwyn Vein" affords an excellent horizon from which 

 the lie of the other strata can be followed in great detail. He 

 has accordingly devoted special attention to tracing the outcrop 

 of this seam, and the trend of the numerous faults which have 

 been met with in working it. He has had occasion to examine 

 a large series of plans of old workings, and to reduce from 

 these the necessary data upon the six-inch Ordnance maps. 

 When these maps are completed, with all the available detailed 

 information, they will probably afford a sufficient and accurate 

 guide to the depth and dip of the various coal-seams over a 

 large part of the area. The information thus worked out, 

 combined with a precise geological mapping of the ground, will 

 prevent the waste of large sums of money in seeking for coal, 

 by showing exactly the limits within which the seams may be 

 looked for, and the depths at which they may be expected. 



Devonian. — Mr. Ussher, in the South of Devonshire, by a sedu- 

 lous scrutiny of the ground, has been enabled to detect the pre- 

 sence of organic remains previously unnoticed, and bytheiraidto 

 distinguish and trace the three great divisions of the Devonian 

 system over the district between Newton Abbot and Plymouth. 

 According to his observations, the following grouping may now 

 be considered as established both by palaeontological and strati- 

 graphical evidence : — 



1. Upper Devonian.— S]iiles, lying on Goniatite Limestone 

 in the Limestone areas, and with local volcanic rocks. 



2. Middle Dtvo7iian. — Slates, Limestones, and Volcanic 

 rocks. The Limestones are developed in a local or sporadic 

 manner, and in the intermediate districts they are replaced by 

 volcanic rocks (the Ashprington Series), while their basement 

 beds are represented by occasional calcareous bands and lenticles 

 in the slate bounding the volcaniCj,series. 



3. Lozver Dcvoman.—Yk^d and Grey Grits, Sandstones, and 



I 



