16 
ORIGIN AND STRUCTURE. 
Various theories had been advanced to account for their origin, 
some of a very unlikely character ; but some pointed to the idea that 
the Sarsens had been derived from the Woolwich and Reading series. 
Mr. Prestwich considered the evidence was in favour of their derivation 
from the mottled clays and sands overlying the chalk, known as the 
Plastic clay, from the Woolwich and Reading series ; and based his 
conclusions mainly on the fact that the sands of these beds were of 
precisely the same mineral character as the blocks themselves—the 
concretionary stone representing, in each case, the component part of 
some portion of the Woolwich and Reading series. 
Mr, Whitaker advanced the opinion, evidently based on sound 
observation, that the sandstone masses, lying on the downs west of 
Marlborough, owed their origin to the Bagshot sands, these sands having 
rested on the chalk of that district, whither they extended beyond the 
London clay, which clay thins off to nothingnear Marlborough. Infurther 
evidence hehad found these sandstones resting on the London clay, which 
intervened between the Bagshot sands and the Woolwich and Reading 
beds. It was remarkable that on this outside edge the sarsens largely 
predominated ; and Mr. Whitaker’s views were further strengthened by 
the fact that the blocks contained no fossils, whereas the underlying 
beds being in some cases fossiliferous, it was likely fossils would 
occasionally be found in them if formed wholly from these. At the 
same time, he coincided with Mr. Prestwich that some of the Conglo- 
merates were referable to the Plastic clay. 
‘ 
The original idea that the greywethers of the Wiltshire downs 
were due to the Bagshots did not, however, rest with Mr. Whitaker, inas- 
much as Smith, the parent of English geology, long ago advanced the 
opinion, that an extensive stratum of sand, containing these stones, 
once overspread the chalk of North Wiltshire. 
Although no fossils had been observed in the greywethers, some 
peculiar root-like holes had the appearance of having come from marine 
plants. Their surfaces were in some instances pitted with basin-like 
hollows, somewhat resembling the depressions made in rocks along the 
sea shores, perhaps by the friction against pebbles lashed into motion 
by the waves. From the uniformity of structure in the Wiltshire 
sarsens it would almost seem as if they were the fragmentary remains 
of a continuous stratum that once had a wide range ; but there could 
