Page:Popular Science Monthly Volume 8.djvu/181

Rh Postponing for the present the more detailed inquiry into the origin of our own Limestone, of which this general survey is the prelude, I pass on to the Permian formation, which rests upon the Carboniferous, and has been upheaved with it, having been deposited previously to the general disturbance that closed the Palæozoic (ancient life) period. Of this Permian formation there are few traces in our part of England; but it has a much greater development in the north, and to it belongs that remarkable bed of Magnesian limestone which comes to the surface in Northumberland and Durham. It is of this stone (selected on account of the durability it has shown in York Minster and other old buildings) that the Houses of Parliament are built. Now, although very few fossils are found in this rock, yet I believe that most geologists would agree that it was originally formed, like limestones generally, by the growth of corals, shells, etc., which separated the carbonate of lime from the sea-water they inhabited; its subsequent conversion into magnesian limestone having been probably effected by the infiltration of water in which magnesia was dissolved. In the Eozoic limestone of Canada, I have myself frequently met with veins of dolomite (magnesian limestone), which retain the general arrangement characteristic of the original shell, although its minute structure has been obliterated by this metamorphic action.

Passing on now to the Secondary or Mesozoic (middle life) series, we find that although the Trias, which is the oldest member of it, is represented in England by sandstones alone, there is an important bed of limestone in Germany called the Muschelkalk (shell-limestone), which is interposed between the lower and the upper New Red Sandstones. This bed derives its name from the fact that it is obviously formed by an aggregation of shells, mingled with other fossils, among which the beautiful Lily Encrinite is one of the most abundant. In the Lias, which overlies the New Red Sandstone, a considerable portion of lime is generally mingled with the clay deposits of which this formation is principally composed; and some of its beds, especially on the northeast of Yorkshire, are almost entirely calcareous. If you walk along the shore between Saltburn and Whitby, and examine the blocks which have fallen from the lias cliffs above, you will find them to be almost entirely made up of fossils; among which Belemnites—conical chambered shells, with solid calcareous "guards," which belonged to animals resembling cuttle-fishes—are specially abundant. And here, as elsewhere, the calcareous matrix in which the fossils are imbedded, though sub-crystalline in some parts, is obviously made up in others of fragments of shell, etc., ground down by the action of the sea in which the deposit was formed. The Lias abounds in the neighborhood of Bristol, and is exposed in many railway-cuttings. These, when in progress some forty years ago, yielded many valuable fossils, especially skeletons of the great Fish-Lizards, which you will see in the Museum of the Bristol Institution. In this neighborhood, also,