LECTURES ON GEOLOGY. ^47 



hood of Birmingham is the transition limestone of Sedgeley, Dud- 

 ley and Walsall. Limestone is also found at Kingsbury, in the 

 Warwickshire coal field, but as it contains no organic remains, it 

 is difficult to fix its place in the series of strata, though from some 

 circumstances it may perhaps be referred to the lias. It is 

 remarkable that the coal of this district reclines upon the transition 

 and not on the carboniferous limestone of the other coal iields of 

 England. 



Lime, next to silex, is the material most universally distributed 

 over the world, more particularly in the strata which succeed the 

 primary. Some geologists have considered that lime might be a 

 secretion of the animals whose remains it contains in such immense 

 quantities ; but a comparison of the lakes in Scotland in which 

 shell marl is now forming, with others where no such formation is 

 taking place, has established the fact that no shell marl is deposit- 

 ed where the water holds no lim.e in solution. To account for the 

 abundance of lime in certain strata, it will be necessary to revert to 

 the primary rocks, of which limestone formed a part. These 

 rocks have been demonstrated to be of igneous origin ; and an 

 examination of recent analogous phenomena, as exhibited in vol- 

 canic countries in the formation of limestone, will afford a clue to 

 the source of the immense deposits of this mineral in the seconda- 

 ry and tertiary strata. Almost all the springs which contain an 

 abundance of carbonic acid, are situated in countries that are, or 

 have been, the seats of volcanic action j and it is well known that 

 water impregnated with this gas will readily dissolve lime. In 

 passing through the calcareous strata the acidulous water dissolves 

 a portion which it brings to the surface, where the gas, escaping, 

 precipitates the lime, thus forming a stalactical deposit. On the 

 banks of the Anio, at Tivoli, this travertine is 600 feet in thickness, 

 and forms an excellent building-stone. In Tuscany the ground is 

 covered by it to a great extent, which would be increased by 

 diffusion if the springs flowed at the bottom of the sea. In such 

 situations shell-fish abound, and their shells are enveloped by the 

 growing mass, which now becomes a perfect shelly limestone. 

 The solution of the lime by the carbonated water in its course will 

 also account for the caverns which are met with in Hmestone rocks. 

 All limestone countries abound in caverns, which are so numerous 

 in Styria as to engulph most of the rivers in some part of their 

 course. That curious animal, the Proteus, one of the real amphi- 

 bia, is found in these subterraneous streams. 



All the limestone strata of this neighbourhood, have a great 

 inclination, particularly at the Wren's Nest, near Dudley, where 

 they resemble the fructrum of a cone, having a dip from 50° to 

 75 ° probably the effect of the same volcanic action that has raised 

 the Rowley Hills. The stone is of two kinds, the black at Sedge- 

 ley and Great Barr, near Walsall, and the white at Dudley and the 

 Wren's Nest. The former is apparently the older formation, and 

 is in thick layers j the latter is in thin strata at top, but it is thicker 

 below. Each layer is separated by a thin parting of marly slate. 



May, 1835. — vol. ii. no. x. 2 k 



