Parr IL. Sor. ii. §2.] DEPOSITS FROM SPRINGS. 353 
results of the application of the microscope to geological inquiry is 
the extent to which it has revealed these all-pervading alterations 
even in what might be supposed to be perfectly fresh rocks. Among 
the silicates the most varied and complex interchanges have been 
effected. Besides the production of calcium carbonate by the de- 
composition of such minerals as the lime-felspars, the series of 
hydrous green ferruginous silicates (delessite, saponite, chlorite, 
serpentine, &c.), so commonly met with in erystalline rocks, are 
usually witnesses of the influence of infiltrating water. The changes 
visible in the olivine of basalt (p. 77) offer instructive lessons of the 
progress of transformation. One further example may be cited as 
supplied by the zeolites, so common in cavities and veins among 
many ancient volcanic and other crystalline rocks. These appear to 
have commonly resulted from the decomposition of felspars or allied 
minerals. Their mode of formation is indicated by the observation 
already cited (p. 300), that Roman masonry at the baths of Plombiéres 
has in the course of centuries been so decomposed by the slow. per- 
colation of alkaline water at a temperature not exceeding 50° C. 
(122° Fahr.) under ordinary atmospheric pressure that various 
zeolitic silicates have been developed in the brick." 
2. Chemical Deposits——Of these by far the most abundant is 
calcium carbonate. ‘The way in which this substance is removed and 
re-deposited by permeating water can be instructively studied in 
the formation of the familiar stalactites and stalagmites beneath damp 
arches and in limestone caves. As each drop gathers on the roof and 
- begins to evaporate and lose carbonic acid, the excess of carbonate 
which it can no longer retain is deposited round its edges as a ring. 
Drop succeeding drop lengthens the ee | 
original ring intoa long pendent tube, Ze 
which, by subsequent deposit inside, 
becomes a solid stalk, and on reaching 
the floor may thicken into a massive 
pillar. At first the calcareous sub- 
stance is soft, and when dry pulveru- 
lent, but it becomes by degrees crys- 
talline. ach stalactite is found to 
possess an internal radiating fibrous 
structure, the fibres passing across 
the concentric zones of growth. The 
stalactite remains saturated with cal- 
-eareous water, and the divergent Fic. 100.—Section oF Parr or 4 
prisms are developed and continued = B*4uAcTIEE. = Macnirip 10 
as radii from the centre of the stalk. é ; 
This process may be completed within a short period. At the 
North Bridge, Edinburgh, for example, which was erected in 1772, 
stalactites were obtained in 1874, some of which measure an inch and 
a half in diameter and possess the characteristic radiating structure. 
1 Daubreée, “ Géologie Expérimentale,” 179, et seq. 
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