326 



FRESH-WATER LAKES. — PEAT. 



The Rock Mill, near Stroud, in Gloucestershire, is built of this stone. 

 In almost all limestone countries, there are instances of calcareous 

 incrustations formed in springs, which have received the name of 

 petrifying wells. 



Thermal waters, that contain calcareous earth in solution, deposit 

 beds of tufa, very rapidly. Nearly the whole bottom of the valley 

 at Matlock Baths, in Derbyshire, is filled with calcareous tufa, form- 

 ing a bed not less than fifty feet in thickness, and half a mile in 

 length. It contains fragments of moss, and some land shells. The 

 horns of a stag were found in excavating this tufa ; it is deposited by 

 the thermal springs, that, every where, gush out from the hill behind 

 the baths. Except the depositions from thermal waters, beds of cal- 

 careous tufa, of any considerable magnitude, are seldom formed on 

 land, but thermal waters have probably been important agents, in the 

 formation of many of the secondary strata at the bottom of the ocean. 

 (See Chap. XV.) 



Mr. Lyell, in the first volume of his "Principles of Geology," has 

 described many depositions of calcareous tufa in the volcanic districts 

 of France and Italy. 



There are depositionsof fresh-water limestone slowly forming in some 

 •of our present lakes. Mr. Lyell, in the "Geological Transactions," 

 1826, describes a small lake about nine miles west of Forfar, in Scot- 

 land. It once extended over two hundred acres, but is now reduced 

 to a peat moss, or swampy hollow in diluvium. The bed of the lake 

 has been, in a great part, excavated for marl ; it contains different 

 strata, of variable thickness. The upper covering is peat, one or 

 two feet thick, under which is shell or rock marl, varying from one 

 to sixteen feet ; quick-sand two feet, and lower shell marl, of a good 

 quality, from one to two feet thick, resting on a bed of fine sand, of 

 variable thickness. The rock marl consists wholly of carbonate of 

 lime; it is hard and compact, and in some parts crystalline. The 

 lower shell marl rarely contains any distinguishable quantity of shelly 

 matter. In the rock marl are found shells of Helices, the Turbo 

 fontinalis, and the Patella lacustris. 



There are remains of land quadrupeds in the shell marl, but not 

 in the rock marl. The rock marl, (it appears from Mr. Lyell's de- 

 scription,) nearly resembles the upper fresh-water limestone in the 

 Paris basin, and, like it, is traversed by tubular cavities. Some part 

 of the rock marl is, however, stated to be a tufaceous limestone. 

 This recent formation of fresh-water limestone, is, in so many res- 

 pects, analogous to the most recent formation of fresh-water strata of 

 the ancient world, that all the particular circumstances described by 

 Mr. Lyell, deserve the careful attention of the geologist. 



Peat is a substance which has been classed with alluvial soils, 

 though it is obviously a vegetable production. Peat formerly cover- 

 ed extensive tracts in England, but is disappearing before the genius 

 of agricultural improvement, which has no where produced more 



