nUl'l'lSlI A.StiOCIATION 51 K K'I'IN( i 
305 
Tlic Uiickucsscs of I lie slrai,a in Glouccslcrshirc an-, derived from an accu- 
rately measured section in the nort h Hank of JU-edon liiU, except, in t he case of 
the inferior Oolite, which is taken from Ijeckhauiptou liill. The thickness of 
strata in Oxfordshire is taken at Stoucslield. It embraced the refi'ion extending 
from Banbury on the north to the range of the Chalk formation south of the 
Thames valley, and from Tarriugdon on the west to Thame on the east, t)xford 
occupying a central position. Mr. Hull then gave a rapid sketch of the 
fornuttions from the Lower Lias up to the Tertiary deposits of the Wool- 
wich and Heading series, which had been surveyed by liiniself and his col- 
leagues, Messrs. VVhitaker and Polwhele, dwelling more especially on the 
position of the iron-beds of the Marlstoue, the distribution of the 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 Shotovcr HUl, which liad 
on a previous day been lucidly described by Professor Phillips, were 
pointed out, and the reasons were stated which had induced the otiicers 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 Fitton, Conybeare, and Phillips in favour of the Wealdeu age of these 
isolated groups. 
SUMMAEY OF PAPBE ON THE GEOLOGICAL SYSTEM OF THE 
CENTRAL SAHARA OF ALGERIA. 
By Rev. H. B. Tkistram, 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 
Imndred 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 
soutli of Tunis, is many feet below the level of the Mediterranean. Prom tliis 
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 Timbuctoo, when 
the traveller will probably lind 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 lou^ inlet from the guK of Cabes to 
Tuggurt was drained and evaporated, leaving its traces in the salt plains, and 
oeclasioual moisture of the Wed Rhix- and Cholt el Melali — the ancient Lake 
Tritonis. 
There are several singular exceptions to the course of the mountain-ranges 
above mentioned, which are generally the local caxises of the oases. 
Thus at Laghonat we find several elliptical basins of dmiinishing 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 fissui'es 
W'hich ])ervade all these super-imposed basins, allow the water to percolate. It 
VOL. III. 2 Q 
