266 
THE GEOLOGIST. 
the author in his former communication showed them to have had at the 
period of the high-level gravels, that is at from 100 to 200 feet above the 
floors of the present valleys, the difference of level then of the upper de- 
posits of Loess to be accounted for will be diminished to 100 or 150 feet, 
— a difference still considerable ; but, 
on the other hand, these extremes are 
O; "y^^znn not always attained. At all events, 
tit '^---- -^/'^^ ^ - ■ this brings the whole of the Loess 
: j^ ^j^^^^'Zr^ — ~1~ within the possible range of inunda- 
; \ ~'ir^-L-S\L - ^ ^ _ tions of the old Pleistocene rivers at 
- ^ different periods according to their 
1. - = ^ ^ — . — age ; the higher beds having been de- 
^ 5' ^" Loess; d a high-level gravel; posited by the overflowings in the 
c c' low-level gravels ; m, n, o, the levels earliest periods and before the exca- 
to wliich the river rose during inundations nation of the present river-valleys, 
at different periods. the lower beds after the present val- 
leys had been formed, but while the 
old meteorological conditions still prevailed. These deposits of Loess thus 
furnish a measure of the volume of water anciently brought down by the 
rivers during floods, and show how very far they exceeded their present 
representatives, and how great must have been their erosive action. Flood- 
deposits will always consist of shingle in the river channels ; sand in 
sheltered places ; fine silt over the area where the flood waters repose. 
In this manner the author considers the high- and low-level gravels and 
the Loess of all the levels to have been formed. 
This flood-origin of the Loess being admitted, it follows that as that de- 
posit is found at 50 to 100 feet above the highest beds of gravel, the floods 
of these periods must have risen like those of the Arctic regions, but to 
even a greater extent (50 to 100 feet), above their summer levels. The 
fluviatile shells found in the gravels and Loess were stated to be identical. 
G-EOLOGiCAL Society of London. — Aj)ril 16, 1862. — 1. " On the 
Position of the Pteraspis-beds, and on the Sequence of the Strata of the 
Old Eed Sandstone Series, in South Perthshire." By Professor Hark- 
ness, F.E.S., P.G.S. 
At the Bridge-of- Allan the lowest beds seen of the Old Bed Series are 
(1) conglomerates of trap-rocks, overlaid by (2) grey sandstone passing up- 
wards into red sandstone. These grey sandstones have afforded to Mr. 
Powrie a Pteraspis (perhaps P. 7'ostratus), and fragments of Cephalaspis 
have also been found in them. Next above come (3) purple shales (at 
Craig Arnhall) ; then the brown sandstones (4) of Downe Castle ; and 
lastly at Laurick, grey sandstones (5) again. These all have a iS".W. dip, 
as seen along the Teith. From about Laurick to Callander the strata are 
best seen in the Keltie burn. They have a S.E. dip, and form the other, 
but steeper, side of a synclinal trough (about twelve miles wide) ; and 
here grey sandstone (5), (4) brown sandstone (at Bracklin Linns), (3) 
purple shale, (2) red and grey sandstone, and lastly (1) a conglomerate 
(here composed of felstone) present an analogous succession to that be- 
tween Laurick and the Bridge- of- AUan. At Callander the conglomerate 
lies almost vertically against the metamorphic Lower Silurian rocks of the 
Grampians, trap-rock intervening. The author estimates that the Old 
Red strata have a thickness of 7000 feet. 
2. " On the Western Extremity of the London Basin ; on the "Westerly 
Thinning of the Lower Eocene Beds in that Basin ; and on the Grey- 
wethers." By William Whitaker, Esq., B.A., F.G.S., of the Geol. Surv. 
Great Britain. 
