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Parr L. Sscr.i.§2] LAVA-STREAMS. — 297 
4 F microscopic crystallites and crystals are usually present and some- 
_ times in prodigious numbers (pp. 104, 141). In most lavas devitri- 
_ fication has proceeded so far before the final stiffening that the 
original glassy magma has passed into a more or less completely 
lithoid or crystalline mass. 
That lava may possess an appreciably crystalline structure while 
still in motion has often been proved at Vesuvius, where well-defined 
_erystals of the infusible leucite may be observed in a molten magma 
of the other minerals, portions of the white-hot rock in this condition 
being ladled out, impressed with a stamp and suddenly congealed. 
The fluxion structure aboye (p. 104) described, furnishes interesting 
evidence of this fact in many ancient as well as modern lavas. 
The crystalline structure appears to be developed in lava under 
some pressure and in presence of the volcanic vapours and gases with 
which the molten rock is impregnated. The rapid escape of these 
vapours may prevent the formation of the erystalline structure and 
leave the lava in the condition of a more or less perfeet glass. This 
may perhaps be the explanation of the vitreous erust on the walls of 
dykes already (pp. 105, 214) referred to. Rocks crystallizing in the 
deeper parts of a voleano appear usually to possess a more coarsely 
crystalline structure than those which crystallize near the surface. 
Temperature of Lava.—lt would be of the highest interest 
and importance to know accurately the temperature at which a lava 
stream first issues. Measurements not altogether satisfactory have 
been taken at various distances below the point of emission where 
4 the moving lava could be safely approached. Experiments made at 
Vesuvius by Scacchi and Sainte-Claire Deville in 1855, by thrusting 
thin wires of silyer, iron, and copper into the lava, indicated a 
temperature uf scarcely 700°C. (1228° Fahr,) Observations of a similar 
kind, made in 1819, when a silver wire ;3,th inch in diameter at once 
- melted in the Vesuvian lava of that year, gave a greatly higher 
temperature, the melting point of silver being about 1800° Fahr. 
But copper wire has also been melted, the point of fusion of 
this metal being about 2204° Fahr. Evidence of the high 
temperature of lava has likewise been adduced from the alteration it 
has effected upon refractory substances in its progress, as where, at 
Torre del Greco, it overflowed the houses, and was afterwards found 
to have fused the fine edges of flints, to have decomposed brass into 
its component metals, the copper actually crystallizing, and to have 
melted silver, and even sublimed it into small octahedral crystals. 
The lava of Santorin has caught up pieces of limestone, and has 
formed out of them nodules containing erystallized anorthite, 
augite, sphene, black garnet, and particularly wollastonite The 
initial temperature of lava, as it first issues from the Vesuvian 
funnel, is probably considerably more than 2000° Fahr. Obviously 
the absorbed water in the white-hot lava must possess as high 
a temperature. The existence of white-hot water, even in rocks 
* Fouqué, op. cit. p. 206. 
Q 2 
