334 Superficial Gravels and Clays . [July, 
At the eastern end of the field the se<Tion shown in Fig. 19 
was exposed. 
About 100 yards north-east of the last section some pits 
have been sunk at various times, opposite Flora Villas, to 
obtain a peculiar sharp dark-yellow sand that is confined to 
a small area. The section exposed at one of these sand-pits 
is shown in Fig. 20. 
There was water at the bottom of this pit. One of the 
workmen, at my request, pushed his spade down about 
18 inches into the soft wet sand, and brought up from that 
depth a great number of shells, amongst which were speci- 
mens of Unio littoralis and U . pictorum with the two valves 
united. Many of the small cyclades also were perfe<5t. 
Section opposite Flora Villas. 
$. Surface soil. n, i. Brown clay. a, 3. Gravel in clay. c. Coarse yellow sand, 
with a band of subangular gravel ; a few broken shells in lower part. /. Sharp 
yellow sand, with mammalian remains and fresh-water shells, common. Base not 
reached. 
This sand is full of shells, amongst which the little Hydrobia 
marginata is abundant. Neither Unto littoralis nor Hydro- 
bia marginata now live in England. Their shells have been 
found in the Lower Thames brick-earths, but were not before 
this discovery known to occur so high up the valley as 
Brentford. 
The superposition of the subangular gravels to the shell- 
bearing sand was clear and decided. The workmen informed 
me that a stiff dark clay came in below the sand, and this is 
doubtless the London Clay. I obtained several mammalian 
bones from the same sand-bed, but they were soft and de- 
composed. The shells also were soft, and those of the Unios 
split up as they dried. Dr. Gwyn Jeffreys has kindly ex- 
amined the shells from this pit, and has given me the 
following list of their names 
