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 de
Beaumont, in their 'Description Geologique 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 interesting account ('The Silurian
System', p. 126) of a fissure 480 feet wide, through which melaphyre has
been ejected, at the coal-mine at Cornbrook, Hoar Edge.
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.
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