299 DYNAMICAL GEOLOGY. [Boos III. 
similarly exposed, with the addition of a little water, was reduced to — 
the state of marble.! These experiments have recently been repeated — 
by G. Rose, who has produced by dry heat from lithographic lime- 
stone and chalk, fine-grained marble without melting. The distinction 
of marble is the independent crystalline condition of its component 
granules of calcite. This structure, therefore, can be superinduced 
by heat under pressure. In nature, portions of limestone which have 
been invaded by intrusive masses of igneous rock have been converted 
into marble, the gradations from the unaltered into the altered rock 
being distinctly traceable, as will be shown in subsequent pages 
(Book IV. Part VIII.). 
Production of Prismatic Structure.—The long-continued high 
temperature of iron-furnaces has been observed to have superinduced 
a prismatic or columnar structure upon the hearth-stones. This fact 
is of interest in geology, seeing that sandstones and other rocks in 
contact with eruptive masses of igneous matter have at various 
depths below the surface assumed a similar internal arrangement 
(Book LY. Part VIITI.). | 
Fusion.—In an interesting series of experiments the illustrious 
De Saussure (1779) fused some of the rocks of Switzerland and — 
France, and inferred from them, contrary to the opinion previously 
expressed by Desmarest,’ that basalt and lava have not been produced 
from granite, but from hornstone (pierre de corne), varieties of — 
“schorl,” calcareous clays, marls, and micaceous earths, and the ~ 
cellular varieties from different kinds of slate.* He observed, how- — 
ever, that the artificial products obtained by fusion were glassy and 
enamel-like, and did not always recall volcanic rocks, though some 
exactly resembled porous lavas. Dolomieu (1788) also contended — 
that as an artificially-fused lava becomes a glass and not acrystalline — 
mass with crystals of easily fusible minerals, there must be some flux — 
present in the original lava, and he supposed that this might be 
sulphur.‘ | 
Sir James Hall, about the year 1790, began an important in- 
vestigation, in which he succeeded in reducing various ancient and 
modern volcanic rocks to the condition of glass, and in restoring 
them, by slow cooling, to a stony state.® Since that time many 
other researches of a more complieated kind have been undertaken, 
especially by Delesse, Daubrée, Deville, Bunsen, Bischof, H. and 
W,. b. Rogers. By these observations it has been abundantly proved 
that all rocks undergo molecular changes when exposed to high 
temperature, that when the heat is sufficiently raised they become 
fluid, that if the glass thus obtained is rapidly cooled it remains 
vitreous, and that, if allowed to cool slowly, a more or less distinct 
Trans. Ioy. Soc. Edin. vi. (1805), p. 101, 121. 
Mem. Acad, Scien. 1771, p. 273. 
De Saussure, Voyages dans les Alpes, edit. 1803, tome i. p. 178. 
Isles Ponces, p. 8 et seq. 
* Trans. Roy. Soc. Edin. v. p. 48. 
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