LiilTISH ASSOCIATION MEETING. 



305 



Tlie tliieknesses of tlie strata in Gloucestershire are derived from an accn- 

 ^•ately measured section in tlie north flank of Bredon Hill, except in the case of 

 ohe inferior Oolite, which is taken from Leckhampton Hill. The thickness of 

 strata in Oxfordshire is taken at Stonesfield. It embraced the rei^ion extending 

 from Banbury on the north to the range of the Chalk formation south of the 

 Thames valley, and from Tarringdon on the west to Thame on the east, Oxford 

 occupying a central position. Mr. Hull then gave a rapid sketch of the 

 formations from the Lower Lias up to the Tertiary deposits of the Wool- 

 wich and Reading series, which had been surveyed by himself and his col- 

 leagues, Messrs. Whitaker and Polwhele, dwelling more especially on the 

 position of the iron-beds of the Marlstone, the distribution of tlie Portland 

 series, and the Lower Cretaceous strata which occur in detached outlying 

 areas of small extent, and evinces the extent of the denudation at several 

 periods. 



The positions of the fresh-water iron-sands of Shotover Hill, which had 

 on a previous day been lucidly described by Professor Phillips, were 

 pointed out, and the reasons were stated which had induced the officers of 

 the Geological Survey to refer these strata provisionally to the Lower 

 Greensand, though it was by no means intended to undervalue the arguments 

 of Pitton, Conybeare, and Phillips in favour of the Wealden age of these 

 isolated groups. 



SUMMAEY OF PAPER ON THE GEOLOGICAL SYSTEM OF THE 

 CENTRAL SAHARA OF ALGERLi. 



By Rev. H. B. Tristham, M.A., F.G.S., &c. 



On leaving the Atlas crest, and descending its southern slopes, we soon 

 come upon the secondary rocks, which are the prevailing formation of the 

 whole country between the Atlas and Laghonat. This district for about four 

 liundred miles due south is rocky, and with mountain-ranges running for the 

 most part in parallel lines north-east and south-west. The southern slopes of 

 the Atlas chain rise from a depression which in several parts, especially to the 

 south of Tunis, is many feet below the level of the Mediterranean. Prom this 

 depression the Sahara is for the most part a system of endless terraces, some 

 of which are only a few miles apart, while others are expanded into plains of 

 from 50 to 100 miles in width, and which, so far as my observations and the 

 information I could gather from native caravans and a trustworthy guide, 

 extended in an unbroken series to within three days' journey of Timbuct'oo, when 

 the traveller will probably find himself on the northern watershed of the valley 

 of the Niger. 



As we advance, on every stage is written the record of the retiring ocean, 

 which gradually, by the elevation of its southern shores, was driven back and 

 back to the northward, till the last long inlet from the gulf of Cabes to 

 Tuggurt was drained and evaporated, leaving its traces in the salt plains, and 

 occasional moisture of the Yf ed Bliir and Cholt el Melah — the ancient Lake 

 Tritonis. 



There are several singular exceptions to the course of the mountain-ranges 

 above mentioned, which are generally the local causes of the oases. 



Thus at Laghonat we find several elliptical basins of diminishing size piled 

 one on another. The lowest and largest rests on the flat surface of the 

 secondary rock, which is the base of the shale system. Several great fissures 

 which pervade all these super-imposed basins, allow the water to percolate. It 



VOL. III. 2 Q 



