HARDWICKE'S SCIENCE-GOSSIP. 



93 



now in operation, varied by climatal chans^es and 

 changes in the height of the district above the sea, 

 was enforced, and the opinion given that the work 

 of elaboration of the Lake-country scenery has been 

 going on ever since Carboniferous or pre-Carboni- 

 ferous times. The lake-hollows represented almost 

 the last rock-shavings removed by Nature's tools. 

 What were the special tools producing these hol- 

 lows ? There being no evidence of their production 

 by marine action or by running water, since they do 

 not lie in synclmal troughs, nor along the lines of 

 iissuring and faulting, and cannot be supposed to 

 be special areas of depression, it remained to see 

 how far Professor Ramsay's theory accounted for 

 their origin. The course of the old Borrowdale 

 glacier was then fully traced out, and the power the 

 numerous tributary glaciers had of helping to urge 

 on the ice over the long extent of flat ground from 

 Seathwaite to the lower end of Bassenthwaite Lake, 

 commented upon. The same was done with regard 

 to the Buttermere and Ireton Glacier, and the 

 depths of the lakes, width and form of the valleys, 

 and thickness of the ice shown by numerous trans- 

 verse and longitudinal sections drawn to scale- 

 When all the evidence was considered— the fact of 

 the lake- hollows under examination being but long 

 shallow troughs, the thickness of the ice which 

 moved along the valleys in which the lakes now lie, 

 the agreement of the deepest parts of the lakes with 

 the points at which, from the confluence of several 

 ice-streams, and the narrowing of the valley, the 

 onward pressure of the ice must have been 

 greatest, — the conclusion was arrived at that Prof. 

 Ramsay's theory was fully supported by these cases, 

 and that the immediate cause of the present lake- 

 basins was the onward movement of the old glaciers, 

 ploughing up their beds to this slight depth. It was 

 pointed out that since thegeneral form of theButter- 

 mere and Crummock valley was that of a round- 

 bottomed basin, as seen in transverse section, the 

 effect of the ice was merely a slight deepening of 

 the basin or the formation of a smaller basin of 

 similar form at the bottom of the larger; whereas 

 in the case of the Derwentwater and Bassenthwaite 

 valley, which in transverse section was a wide flat- 

 bottomed pan, the action was to form long shallow 

 grooves at the bottom of the pan. This consider- 

 ation was thought to explain the fact of the greater 

 depth of Buttermere and Crummock than of 

 Derwentwater and Bassenthwaite, although the 

 size and thickness of the old glacier in the former 

 case was probably less than in the latter. 



Geological Notes on a Journey from Algiers 

 TO THE Sahara.— George Maw, Esq., F.L.S., 

 E.G.S., has just read a paper before the Geological 

 Society of London, on the above subject. The 

 author commences by describing the details ob- 

 served on his journey from Algiers to L'Aghouat, on 



the borders of the Sahara. The distance traversed 

 was 285 miles, or about 210 miles in a straight line, 

 and in a direction nearly north and south. No erup- 

 tive rocks were observed. The oldest rock is a boss 

 of mica-schist and gneiss behind the city of Algiers ; 

 it forms a low anticlinal, with the N. and S. strike. 

 The pass througii the gorge of the Chiffa in the 

 Lesser Atlas shows hard slaty rocks dipping S. at a 

 high angle ; they are repeated as an anticlinal, on 

 the south side of the higher part of the Tell plateau, 

 and are probably ]\Iesozoic. In the plain separating 

 the Tell from the Hants Plateaux, and on the so uth 

 side of the latter, red and yellow sandstones form 

 anticlinals ; these rocks resemble the Bunter in 

 mineral characters, and are overlain by red marls 

 resembling the Keuper. In the northern escarpment 

 of the Hants Plateaux saliferous marls are exposed, 

 interstratifled between the sandstones and below the 

 red and grey marls. Crystals of salt and gypsum 

 are intimately mixed with the grey marls, and the 

 so-called " Rochers de Sel " are capped with great 

 blocks of rock tumbled about in confusion, the 

 position of which the author ascribes to the failure 

 of support due to the solution of the salt in the 

 underlying salt marls. A thin series of bright red 

 and green marls is seen to overlie the Red Sand- 

 stones in several places; and above this is an immense 

 series of dark grey marls, interstratifled with argil- 

 laceo-calcareous bands, forming a great synclinal of 

 the Hants Plateaux, and a contorted mass on the 

 Tell plateau. These are probably cretaceous. At 

 L'Aghouat they are overlain by fossiliferous beds, 

 probably of Miocene age. Other tertiary beds 

 observed are soft yellow calcareous freestones on 

 the flanks of the promontory of Algiers and of 

 the Lesser Atlas, and some red and grey marls and 

 ferruginous freestoue'capping the Tell plateau, the 

 former at a height of 100—900 feet, and the latter 

 of 2,500-4000 feet above the sea-level. The plain 

 of the Mitidja, between the Lesser Atlas and 

 Algiers, consists of grey loam with shingle-beds of 

 post-tertiary age. A similar loam covers the great 

 plain of the northern Sahara, and rises to a height 

 of 2,700 feet. Raised beaches occur on the coast 

 up to an elevation of GOO feet above the sea-level ; 

 and similar beaches are found inland, south of the 

 Tell plateau, at a height of 2,000 feet. The oldest 

 land in the line of section is the anticlinal of mica- 

 schist near Algiers, the strike of which is nearly 

 at right angles to that of the other rocks. The 

 upheaval of the Mesozoic rocks was contempo- 

 raneous with the first upheaval of the Lesser Atlas ; 

 it was followed by a long period of denudation, and 

 this by a subsidence of at least 3,000 feet in 

 Tertiary times, during which the Miocene deposits 

 were formed. The Tell plateau was thus elevated 

 at least 4,000 feet, and the district north of the 

 Lesser Atlas at least 1,000 feet, the north face of 

 those mountains probably marking a post-tertiary 



