The most cursory observation of the surface of the earth shows that it is composed of distinct substances, such as clay, chalk, lias, limestone, coal, slate, sandstone, &c.; and a study of such substances, their relative position and contents, has led to the conviction that these external parts of the earth have acquired their present condition gradually, under a variety of circumstances, and at successive periods, during which many races of animated beings, distinct both from those of other periods and from those now living, have successively peopled the land and the waters; the remains of these creatures being found buried in many of the layers or masses of mineral substances, forming the crust of the earth.
The object of the Islands in the Geological Lake is to demonstrate the order of succession, or superposition, of these layers or strata, and to exhibit, restored in form and bulk, as when they lived, the most remarkable and characteristic of the extinct animals and plants of each stratum.
The series of mineral substances and strata represented in the smaller island have been called by geologists “secondary formations,” because they lie between an older series termed “primary,” and a newer series termed “tertiary:” the term “formation” meaning any assemblage of rocks or layers which have some character in common, whether of origin, age, or composition.[1]
Following the secondary formations as they descend in the earth, or succeed each other from above downwards, and as they are shown, obliquely tilted up out of their original level position from left to right, in the Secondary Island, they consist: 1st, of the Chalk or Cretaceous group; 2nd, the Wealden; 3rd, the Oolite; 4th, the Lias; and 5th, the New Red Sandstone.
THE CHALK.
The chalk formations or “cretaceous group of beds” include strata of various mineral substances; but the white chalk which forms the cliffs of Dover and the adjoining coasts, and the downs and chalk quarries of the South of England, is the chief and most characteristic formation. Chalk, immense as are the masses in which it has been deposited, owes its origin to living actions; every particle of it once circulated in the blood or vital juices of certain species of animals, or of a few plants, that lived in the seas of the secondary period of geological time. White chalk consists of carbonate of lime, and is the result of the decomposition chiefly of coral-animals (Madrepores, Millepores, Flustra, Cellepora, &c.), of sea-urchins (Echini), and of shell-fishes (Testacea), and of the mechanical reduction, pounding, and grinding of their shells. Such chalk-forming beings still exist, and continue their operations in various parts of the ocean, especially in the construction of coral reefs and islands.
Every river that traverses a limestone district carries into the sea a certain proportion of caustic lime in solution: the ill effects of the accumulation of this mineral are neutralised by the power allotted to the above-cited sea-animals to absorb the lime, combine it with carbonic-acid, and precipitate or deposit it in the condition of insoluble chalk, or carbonate of lime.
The entire cretaceous series includes from above downwards:
Maestricht beds of yellowish chalk.
Upper white chalk with flints.