Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/235

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225
Mr. Webster on the Strata lying over the Chalk.

a granular quartz. It has more the appearance of an original formation, or peculiar crystallization of siliceous matter analogous to that of sugar, than to a substance composed of the detritus of other rocks.

Numerous large and loose masses of this rock lie scattered over the surface of the chalk country, particularly in Berkshire and Wiltshire, but a bed or continuous stratum of it has not yet been observed. These stones were much employed by our ancestors in building, and before the ground was cleared for the purposes of agriculture they were much more numerous than at present. The huge erections of Stonehenge, which have so much exercised the conjectures of our antiquaries, are chiefly[1] composed of it, and the blocks were no doubt found on the spot.

This granular quartz bears a close resemblance to the siliceous cement of the Hertfordshire pudding-stone, which also is often found in loose masses above the London clay. There appears no necessary connexion between the pebbles of this beautiful conglomerate and their cement, but the dates of their origins were very different; the siliceous deposition, when it did not envelope any foreign substance, forming the rock called the grey weather; and when it fell among pebbles of any kind, composing a pudding stone. Accordingly we sometimes find in the grey weathers common chalk flints.[2]

  1. It is not a little singular that some of the smaller upright stones of Stonehenge consist of a sort of greenstone, and must therefore have been brought from very great distance, no such rock occurring in the neighbourhood.
  2. Specimens of these collected in the neighbourhood of High Wycombe, by the Hon. Henry Grey Bennett, Pres. G. S. are deposited in the Museum of the Society.

Vol. II.2 f