Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/197

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Mr. Webster on the Strata lying over the Chalk.

In the Isle of Wight basin, both in Dorsetshire and Alum bay, beds of iron stone and ferruginous sand occur connected with this clay, and generally lying over it. Considerable rocks of it are seen about Studland, and the Druidical monument, called the Agglestone;[errata 1] near that place is a huge block of this bed.

A stratum of sand, containing green particles, frequently occurs near the chalk. It is seen in Alum bay without fossils; at Reading it is found containing oyster shells. This green sand is easily distinguished from that below the chalk, as it is never indurated.

2. London or Blue Clay. The stratum which has received this dedenomination is found immediately under the gravelly soil on which the metropolis is situated. Of all the strata over the chalk in this country, it is of the greatest extent and thickness: and the number, beauty, and variety of the organic fossils which it contains, renders it the most interesting and the most easily recognizable.

It consists generally of a blackish clay, sometimes very tough, at other places mixed with green earth and sand, or with calcareous matter.

It contains also numerous flat spheroidal nodules of indurated marl, or argillaceous limestone, which lie in regular horizontal layers, at unequal distances, generally from four to ten feet apart. These nodules are well known by the name of Ludus Helmontii, or Septaria, from their being divided across by partitions or veins of calcareous spar, which are generally double. In their cavities are frequently found crystals of calcareous spar and of sulphat of barytes. The septaria are surrounded by crusts which contain a smaller proportion of carbonat of lime than the central part. They often include organic remains.

Besides the clay, marl, sand, and carbonat of lime, of which the main body of this stratum consists, several other substances are dis-


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