Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/102

 98 which this clay is easily divided, are often entirely covered with them as with small seeds. The same shells occur also in the Hastings sand and sandstone, in the Sussex marble, and in the Purbeck limestone, all of which were deposited during the same geological epoch in an ancient lake or estuary, wherein strata of this formation have been accumulated to the thickness of nearly 1000 feet. (See Dr. Fitton's Geol. sketch of Hastings, 1833, p. 68.)

We have similar evidence of the long duration of time, in another series of Lacustrine formations, more recent than the chalk, viz. in the great freshwater deposites of the tertiary period in central France; here the district of Auvergne presents an area of twenty miles in width, and eighty miles in length, within which strata of gravel, sand, clay, and limestone have been accumulated by the operations of fresh water, to the thickness of at least seven hundred feet. Mr. Lyell, in his Principles of Geology, 3d ed. vol. iv. p. 98, states that the foliated character of many of the marly beds of this formation is due to the presence of countless myriads of similar exuviæ of the Cypris which give rise to divisions in the marl as thin as paper. Taking this fact in conjunction with the habit of these animals to moult and change their skin annually, together with their shell, he justly observes that a more »convincing proof of the tranquillity of the waters, and of the slow and gradual process by which the lake was filled up with fine mud cannot be desired.

Another proof of the length of time that must have elapsed during the deposition of these tertiary fresh water formations in Auvergne, is afforded near Cleremont by the occurrence of beds of limestone, several feet in thickness, almost wholly made up of the Indusiæ, or Caddis-like coverings, resembling the cases that enclose the larvæ of our common May-fly.

Mr. Lyell states that a single individual of these Indusiæ is often surrounded by no less than a hundred minute shells