Page:Popular Science Monthly Volume 8.djvu/185

Rh which has been the great agent in the metamorphism of many calcareous rocks, whereby their texture has been entirely changed, while their composition remains unaltered; and it acts with augmented potency where heat and pressure concur to increase it. Of this we have an example in the action of hot springs highly charged with carbonic acid, such as we often find in volcanic localities; it is to such that the formation of the "travertine" limestone of Italy is due, the carbonate of lime being slowly deposited almost in the condition of marble, when the excess of carbonic acid is disengaged, and the water is dispersed in vapor, by free exposure to air. We have familiar examples of this, on a more limited scale, in the formation of the "stalactites" which hang from the roofs of caves in limestone rocks (as at Cheddar), and in the "stalagmitic" crust formed by their droppings on the floors.

Those who have had opportunities of observing the changes which have taken place in the condition of recent corals that have been upheaved by volcanic action above the level of the sea, in the "area of elevation" to which Mr. Darwin drew attention forty years ago, assure us that their texture is often so changed, that detached pieces of them could not be distinguished from pieces of sub-crystalline limestone. I well remember having first learned this from Mr. S. Stutchbury, who was the curator of the museum here when I was a youth, and who was the first to observe the ring of upraised coral which encircles the cone of the great volcano of Tahiti, and which belongs to the same type as that now forming reefs around the coast of that island. He told me that some specimens of it, which he had collected and brought home, were treated by his brother, a professed mineralogist, as specimens of carboniferous limestone. The formation of oolites, again, may be studied at the present time. When a bed of calcareous sand, formed by the wearing down of shells or corals, is raised above the sea-level, and is penetrated by rain-water charged with carbonic acid, this, dissolving the carbonate of lime of the surface-layer, deposits it again around the grains of the deeper layers, which it invests with concentric coats. Such oolites present themselves in various geological epochs, indicating the similarity of the past and present conditions. There are oolitic beds, for example, in the Clifton rocks; and we thus know that these must have been shore formations; while other beds seem to have been deep-sea deposits, resembling the Globigerina mud of the present Atlantic sea-bottom. For there is in Russia a very extensive bed of limestone belonging to the carboniferous series, which is as completely composed of Fusuinæ (an extinct type of foraminifers about the size of a sugar-plum) as the nummulitic limestone is of nummulites. In the clay-seams, again, which we sometimes find inter posed between beds of pure limestone, numerous Foraminifera are found well preserved, of which some belong to types still living; and my friend Mr. H. B. Brady, of Newcastle, who has been lately making a microscopic study of the Carboniferous Foraminifera, has found their