ON A PIECE OF LIMESTONE. 17^ 



which has been the great agent in the metamorphism of many calca- 

 reous rocks, whereby their texture has been entirely changed, while 

 their composition remains unaltered ; and it acts with augmented po- 

 tency 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 up- 

 heaved 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, dis- 

 solving the carbonate of lime of the surface-layer, deposits it again 

 around the grains of the deeper layers, which it invests with con- 

 centric 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 FusuUnce (an extinct type of foraminiferji 

 about the size of a sugar-plum) as the nuramulitic limestone is of num 

 mulites. 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 



