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304 DYNAMICAL GEOLOGY. [Book Ill. 
in limestone converted into marble; in others it has involved the 
introduction of mineral solutions, and the partial or complete trans- 
formation of the original constituents, whether crystalline or elastic, 
into new crystalline mineralsf Quartz-rock is evidently a compacted 
sandstone, either hardened by mere pressure, or most frequently by 
the deposit of silica between its granules, or a slight solution of 
these granules by permeating water so that they have become 
mutually adherent. A clay-slate is a hardened, cleaved, and somewhat 
altered form of muddy sediment, which on the one hand may be 
found full of organic remains like any common shale, while on the 
other it may be traced becoming more and more crystalline 
until it passes into chiastolite-slate, or some other crystalline rock. 
Yet remains of the fossils may be obtained even in the same hand- 
specimens with crystals of andalusite, garnet, or other minerals. The 
caleareous matter of corals is sometimes replaced by hornblende, 
garnet, and axinite without deformation of the fossils. * 
A few illustrative examples of metamorphism may be given here; 
the structure of metamorphic rocks, with the phenomena of “ regional ” 
and “contact” metamorphism, will be discussed in Book IV., 
Part VIII. 
Production of Marble from Limestone.—One of the most 
obvious cases of alteration—the conversion of ordinary limestone 
into crystalline saccharoid marble—has been already (p. 291) 
referred to.? The calcite having undergone complete transformation, 
its original structure, whether organic or not, has been effaced, and a 
new structure has been developed consisting of an aggregate of 
minute rounded grains, each with an independent crystalline 
arrangement. The production of a crystalline structure in amorphous 
calcite, may be effected by the action of mere meteoric water at or 
near the surface (ante, p. 166 and postea, p.353). But the generation 
of the peculiar granular structure of marble always demands heat 
and pressure and probably usually the presence of water ; the details 
of the process are, however, still involved in obscurity. We know 
that where a dyke of basalt or other intrusive rock has involved 
limestone, it has sometimes been able to convert it for a short 
distance into marble. The heat (and perhaps the moisture) of the 
invading lava have sufficed to produce a granular structure, which 
even under the microscope is identical with that of marble. The — 
conversion of wide areas of limestone into marble is a regional 
metamorphism associated usually with the alteration of other 
sedimentary masses into schists, &c. 
Dolomitization.—Another alteration which from the labours of 
Von Buch received in the early decades of this century much attention 
from geologists is the conversion of ordinary limestone into dolomite. 
Some dolomite appears to be an original chemical precipitate 
from the saline water of inland seas (Part II. Sect. ii. § 4). But 
1 Ann. des. Mines, 5me ser, xii. p. 318. 
* See also “ Marmarosis”’ in Book 1V. Part VIII. 
