Some groups of dolerite and trachyte indicate p 258 a certain degree of basaltic fluidity; others, which have been expanded into vast craterless domes, appear to have been only in a softened condition at the time of their elevation. Other trachytes, like those of the Andes, in which I have frequently perceived a striking analogy with the greenstones and syenitic porphyries (which are argentiferous, and without quartz), are deposited in the same manner as granite and quartzose porphyry.
Experiments on the changes which the texture and chemical constitution of rocks experience from the action of heat, have shown that volcanic masses* (diorite, augitic porphyry, basalt, and the lava of AEtna) yield different products, according to the difference of the pressure under which they have been fused, and the length of time occupied during their cooling; thus, where the cooling was rapid, they form a black glass, having a homogeneous fracture, and where the cooling was slow, a stony mass of granular crystalline structure.
[footnote] *Sir James Hall, in the 'Edin. Trans.', vol. v., p. 43, and vol. vi., p. 71; Gregory Watt, in the 'Phil. Trans. of the Roy. Soc. of London for' 1804, Part ii., p. 279; Dartigues and Fleurieu de Bellevue, in the 'Journal de Physique', t. lx., p. 456; Bischof, 'Warmelchre', s. 313 und 443.
In the latter case, the crystals are formed partly in cavities and partly inclosed in the matrix. The same materials yield the most dissimilar products, a fact that is of the greatest importance in reference to the study of the nature of erupted rocks, and of the metamorphic action which they occasion. Carbonate of lime, when fused under great pressure, does not lose its carbonic acid, but becomes, when cooled, granular limestone; when the crystallization has been effected by the dry method, saccharoidal marble; while by the humid method, calcareous spar and aragonite and produced, the former under a lesser degree of temperature than the latter.*
[footnote] *Gustav Rose, in Poggend., 'Annalen.' bd. xliii., s 364.
Differences of temperature p 259 likewise modify the direction in which the different particles arrange themselves in the act of crystallization, and also affect the form of the crystal.*
[footnote] *On the dimorphism of sulphur, see Mitscherlich, 'Lehrbuch der Chemie', 55-63.
Even when a body is not in a fluid condition, the smallest particles may undergo certain relations in their various modes of arrangement, which are manifested by the different action on light.*
[footnote] *On gypsum as a uniaxal crystal, and on the sulphate of magnesia, and the oxyds of zinc and nickel, see Mitscherlich, in Poggend., 'Annalen.' bd. xi., s. 328.
The phenomena presented by devitrification, and by the formation of steel by cementation and casting — the transition of the fibrous in the granular tissue of the iron, from the action of heat* and probably, also, by regular and long-continued concussions — likewise throw a considerable degree of light on the geological process of metamorphism.