110 Geology of Wiltshire. 
sands. 5th. Bracklesham sands. In our county they are generally 
thin, often reduced to a mere sprinkling of flinty pebbles over the 
surface, and rarely containing any fossils, except occasional beds of 
an oyster (Ostrea Bellovacina) not very unlike our edible species. 
In some of the hollows of the downs about Marlborough and 
Kennet, are found those well known and singular masses of hard, 
white, siliceous grit, almost saccharine (i.e. crystalline like sugar) 
known by the provincial names of Sarsen-stones, Grey-wethers, or. 
Druid sandstone, from their having been employed in the construc- 
tion of the supposed Druidical fanes of Abury and Stonehenge. 
It is the opinion of Mr. Prestwich, that they are relics of concre- 
tionary masses formed in beds of pure white sand belonging to the 
Lower Tertiary strata that formerly covered the chalk of this 
neighbourhood, these masses having been left stranded in the hol- 
lows when the the looser materials of the beds were swept away. 
Other geologists think they were similarly left from the Bagshot 
sands, a higher series of beds in the tertiary scale. They are al- 
most peculiar to the Wiltshire Downs. In some places, such as 
Clatford and Dean bottoms, there is a remarkably thick trail of 
them. 
On the whole, we may safely conclude, that these Eocene beds 
were deposited in littoral and shallow water, and the chalk on which 
they rest must have been brought up very near the surface of the 
sea before, or about, the time of their formation. j 
These are the most recent marine strata occurring within the 
county. Since its elevation into dry land, the bottoms of its val- 
leys have been more or less covered or filled with drift or alluvial 
deposits of gravel, flint and chalk rubble, and brick earth in the 
chalk districts, of these, together with oolitic gravel in the north. In 
them are found many land and fresh-water shells, bones of red deer, 
ox and horse, and even of elephant, hippopotamus, and rhinoceros, 
probably some of the earliest inhabitants of the land. This is the 
‘Mammalian Drift’ described by Mr. Cunnington in a recent num- 
ber of this Magazine; to which I refer my readers for a more full 
and interesting account of its distribution and contents. 
To him and other scientific readers of this mere general sketch of 
