236 Geological Society :— 
hood of Oxford, where it is difficult to trace out correctly the limits 
of the Lower Cretaceous beds. The Oolitic rocks having been 
deposited whilst the relative position of the land and sea was being 
changed, many of the deposits are subject to local limitation; thus 
the Coralline, Oolitic, and the Calc-grit die out rapidly, and the 
Kimmeridge Clay comes to rest on the Oxford Clay. It is on the 
surface formed by these irregular beds, and that surface considerably 
denuded, owing to elevations before the Oolitic period was ended, 
that the Lower Cretaceous beds have been laid down. From their 
close propinquity, the sand-beds of different ages, when without 
fossils, are scarcely to be defined as Oolitic or Cretaceous, and where 
one clay lies upon a similar clay, the occurrence of fossils only can 
secure their distinction. 
The Farringdon sands, the sands of Shotover Hill, and those near 
Aylesbury, are still open to research,—their Lower Greensand 
characters not having been clearly established. At Culham, a few 
miles south of Oxford, ua clay-pit is worked, which presents, at the 
top, 3 feet of gravel; next about 20 feet of Gault with its peculiar 
fossils; then 9 feet of greenish sand, with a few fossils ; and lastly 
23 feet of Kimmeridge Clay, with its peculiar Ammonites and other 
fossils. In winter the clay-pit, being wet, offers little evidence of 
any distinction between the upper and the lower parts of the clay ; 
but in summer the Gault and its fossils are more easily recognized. 
The intervening sand contains Thracia depressa, Cardium striatulum, 
and an Ammonite resembling one found in the Kimmeridge Clay. 
Although this sand at first sight resembles the Lower Greensand, 
yet it is probably more closely related to the Kimmeridge Clay. 
Puzzling as this sand is in the pit, another enigma is offered 
by the railway-section at Culham, where the Kimmeridge Clay 
is overlaid by a sand perhaps equivalent to that of Shotover 
Hill, not that of the clay-pit; whilst the Gault, which lies on it 
unconformably, can be connected with that of the clay-pit. At 
Toot Baldon also, though Lower Greensand probably caps the hill, 
yet an Oolitic Ammonite was found on the eastward slope of the 
hill, in a ferruginous sand, lying conformably on the Kimmeridge 
Clay. From these and other instances the difficulty of mapping the 
country geologically may be shown to be very great,—the sands of 
any one bed differing in colour from green to red, according to the 
amount of oxidation produced by exposure and other causes; and if 
fossils are absent, the Portland Sand and the Lower Greensand, 
lying against each other, may never be defined. From the great 
and irregular denudation, too, of the rocks, and the unequal depo- 
sition of many of the beds, it will prove a difficult problem to trace 
the several sands and define their age,—a problem to be solved only 
by close perseverance and strict search for organic remains. 
2. «On the Association of the Lower Members of the Old Red 
Sandstone and the Metamorphic Rocks on the Southern Margin of 
the Grampians.” By Prof. R. Harkness, F.R.S., F.G.S. 
The area to which this paper referred is the tract lying between 
