Geological Notes on a Journey from Algiers to the Sahara. 75 



The beds which support Geological Basins frequently thin towards 

 the centres of those basins, thus producing, wholly or in part, the 

 basined form of the strata. It was, however, shown that the beds of 

 the basins themselves frequently thicken towards thecentre of the basins. 



February 25th. — John Evans, Esq., F.B.S., President, in the Chair. 



The following communications were read : — 



1. " Geological Notes on a Journey from Algiers to the Sahara." 

 By George Maw, Esq., F.L.S., F.G.S. 



The author commences by describing the details observed 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 eruptive 

 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 a 

 N". and S. strike. The pass through 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 Mesozoic. In the plain 

 separating the Tell from the Hauts Plateaux, and on the south 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 Hauts Plateaux saliferous marls are exposed, interstratified 

 between the sandstones and below the red and grey marls. Crystals 

 of salt and gypsuni are intimately mixed with the grey marls ; and 

 the so-called " Bochers 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 Bed Sandstones in several places ; and above this 

 is an immense series of dark grey marls, interstratified with argil- 

 laceo-calcareous bands, forming a great synclinal of the Hauts 

 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 freestone capping the Tell plateau, the former at a height 

 of 100-900 feet, and the latter of 2500-4000 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 2700 feet. Baised beaches occur on the coast 

 up to an elevation of 600 feet above the sea-level; and similar beaches 

 are found inland, south of the Tell plateau, at a height of 2000 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 

 contemporaneous with the first upheaval of the Lesser Atlas ; it was 

 followed by a long period of denudation, and this by a subsidence of 



