214 Dr. Cooper on Volcanoes 
5. The cause of the columnar and prismatic figure of vol- 
canic basalt is as yet undetermined. 
Basalt affords strong suspicion of its having been formed 
out of primitive, or transition Horablende rocks : from its 
appearance—from hornblende accompanying it—from its 
analysis Lavas however, have been found by Cordier after 
mechanical examination by means ofa microscope, to be 
composed chiefly of felspar, and black augit or pyroxene 
and oxyd of Iron. This mineral (augit) of other colours 
than black, is found occasionally in primitive strata, but the 
black crystals of augit seem, to be peculiar to Lava, and 
characterize it. ence, augit has been considered as an 
igneous product of volcanic action; a conjecture that de- 
rives strength from the fact that Dr. Thomson found crystals of 
augit sublimed in a chimney of a house on Vesuvius that 
had been exposed to a current of Lava. Insulated crys- 
tals of augit, are thrown out by volcanoes in prodigious 
abundance. Breisl. 675.50, near Rome and at Frescati 
voleanoe of Mons Latiali: 
on the average to about 20 per cent: it is frequently _titani- 
ferous, indicative of a primitive origin. When the Felspar 
prevails in Lavas, the fusion is comparatively more easy. 
Lavas may be considered generally as felspathic, (‘Trachy- 
tes) or augitic and amphibolic. Lavas are also, fibrous ob- 
sidian or pumice; or they are earthy and lithoid ; or fria- 
ble and hard as Tufas ; or decomposed into an ocreous sub- 
stance as Puzzuolana or ‘Tarras; or into an earthy blackish 
argillaceous substance as the wacke of the Germans. In 
fact, Lavas will have different appearances in proportion to 
the degree of heat they have undergone—the continuance of 
the heat—and the nature of the stone itself and its relations 
to caloric. vas consisting of Felspar, Augit and Iron, as 
their general component parts, have been observed also as 
containing or enveloping, crystals of mica, of Hornblende, 
Petrosilex 
Pe : 
Fragments of Granite and other primitive rocks not ap- 
parently acted on by fire. 1. Spalanz. trav. 78. Crystals 
se ener etteenamantrramenna a nie ae 
sesame rss ences nn nme mca 
[Fe 
