258 COSMOS. 



licate 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 fre- 

 quently perceived a striking analogy with the greenstones and 

 eyenitic 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 chem- 

 ical constitution of rocks experience from the action of heat, 

 have shown that volcanic masses* (diorite, augitic porphyry, 

 basalt, and the lava of ^Etna) yield different products, accord- 

 ing to the difference of the pressure under which they have 

 been fused, and the length of time occupied during their cool- 

 ing ; 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. 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 import 

 ance 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, sac- 

 charoidal marble ; while by the humid method, calcareous 

 spar and aragonite are produced, the former under a lesser de- 

 gree of temperature than the latter. t Differences of temper- 

 granite occurs, expresses the general or leading character of the whole 

 formation. But its aspect at some places leads to the belief that it wa3 

 occasionally more fluid at the period of its eruption. The description 

 given by Rose, in his Reise nach dem Ural, bd. i., s. 599, of part of the 

 Narym chain, near the frontiers of the Chinese territories, as well as the 

 evidence afforded by trachyte, as described by Dufrenoy and Elie da 

 Beaumont, in their Description Gtologique de la France, t. i., p. 70. 

 Having already spoken in the text of the narrow apertures through 

 which the basalts have sometimes been effused, I will here notice the 

 large fissures, which have acted as conducting passages for melaphyres, 

 which must not be confounded with basalts. See Murchison's inter- 

 esting account (The Silurian System, p. 12GJ of a fissure 480 feet wide, 

 through which melaphyre has been ejected, at the coal-mine at Corn- 

 brook. Hoar Edge. 



* 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 

 180-1, Part ii., p. 279 ; Dartigues and Fleurieu de Bellevue, in the Jour- 

 nal de Physique, t. lx., p. 45(> ; Bischof, Wdrmclehrc, B. 313 und 443. 



