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354 DYNAMICAL GEOLOGY. [Boox II. 
It is doubtless by an analogous process that limestones, originally — 
composed of the débris of calcareous organisms and interstratified 
among perfectly unaltered shales and sandstones, have acquired a 
crystalline structure.’ 
Calcareous springs deposit abundantly a precipitate of carbonate 
of lime upon mosses, twigs, leaves, stones and other objects. The 
precipitate takes place when from any cause the water parts with — 
carbonic acid. ‘This may arise from mere evaporation, but is pro- 
bably mainly caused by the action of bog mosses and water plants, 
which, decomposing the carbonic acid, cause a crust of carbonate of 
lime to be deposited round their stems and branches (postea, p. 461). 
Hence calcareous springs are popularly called “ petrifying,” though 
they merely encrust organic bodies and do not convert them into 
stone. Calc-sinter, as this precipitate is called, may be found in 
course of formation in most limestone districts, sometimes in masses 
large enough to form hills and compact enough to furnish excellent 
building stone. The travertine of Tuscany is deposited at the Baths 
of San Vignone at the rate of six inches a year, at San Filippo 
one foot in four months. At the latter locality it has been piled 
up to a depth of at least 250 feet, forming a hill a mile and a 
quarter long and the third of a mile broad.” 
Chalybeate springs give rise to a deposit of hydrous peroxide 
of iron. This has already been referred to as a yellow and brown 
deposit along the channels of the water. But in undrained districts 
of temperate latitudes in Northern Europe and America much iron 
is also deposited beneath soil which rests on a retentive subsoil. - 
When the descending water is arrested on this subsoil the iron, in — 
solution as organic salts that oxidize into ferrous carbonate, is 
gradually converted into the insoluble hydrous ferric oxide which is 
precipitated and forms a dark ferruginous layer known to Scottish — 
farmers as “moorband pan.” So effectually does this layer interrupt 
the drainage that the soil remains permanently damp and unfertile. 
But when the “pan” is broken up and spread over the surface it 
quickly disintegrates, and improves the soil, which can then be 
properly drained (postea, p. 463). 
Siliceous springs form important masses of various sinters round 
the point of outflow. The basins and funnels of geysers have 
already been described (p. 236). One of the sinter-beds in the Iceland 
geyser region is said to be two leagues long, a quarter of a league 
wide, and a hundred feet thick. Enormous beds of similar material 
have been formed in the Yellowstone geyser region. Such accumu- 
lations point to proximity to volcanic centres, or at least to the 
escape of hot water to the surface. 
* Sorby, Address to Geological Society, Q. J. Geol. Soc. 1879, p. 42, et seg. The 
finely fibrous structure seen in caleedony under the microscope with polarized light 
passes in a similar way through the bands of growth of pebbles. 
* Lyell, “ Principles,” i. p. 402. The student will find much detail regarding the 
abstraction and deposit of carbonate cf lime by subterranean water in a paper by Senft, 
“Die Wanderungen und Wandelungen des kohlensauren Kalkes,” Z. Deutsch. Geol. 
Ges. xiii. p. 263. 


