
~ 
os 
Parr L Secr.i.§2.] LAVA-STREAMS. | 231 
z | quietly did it advance that Breislak could sail round it in a boat 
and observe its progress. 
By the outpouring of lava two important kinds of geological 
change are produced. (1) Stream-courses, lakes, ravines, valleys, in 
short all the minor features of a landscape, may be completely over- 
_whelmed under a thick sheet of lava. The drainage of the 
district being thus effectually altered, the numerous changes which 
flow from the operations of running water over the land are 
arrested and made to begin again in new channels. (2) Con- 
siderable alterations may likewise be caused by the effects of the 
heat and vapours of the lava upon the subjacent or contiguous 
ground. Instances have been observed in which the lava has 
actually melted down opposing rocks, or masses of slags on its own 
surface. Interesting observations, already referred to, have been 
made at Torre del Greco under the lava-stream which overflowed 
part of that town in 1794. It was found that the window-panes of 
the houses had been deyitrified into a white, translucent, stony 
substance ; that pieces of limestune had acquired an open, sandy, 
granular texture, without loss of carbon dioxide, and that iron, brass, 
lead, copper, and silver objects had been greatly altered, some of the 
metals being actually sublimed. We can understand, therefore, that, 
retaining its heat for so long a time, a mass of lava may induce many 
crystalline structures, rearrangements, or decompositions in the rocks 
over which it comes to rest, and proceeds slowly to cool. This is 
a question of considerable importance in relation to the behaviour 
of ancient lavas which have been intruded among rocks beneath the 
surface, and have subsequently been exposed (Book IV. Part VIL.). 
But on the other hand, the exceedingly trifling change produced, 
even by a massive sheet of lava, has often been remarked with 

astonishment. On the flank of Vesuvius vines and trees may be 
seen still flourishing on little islets of the older land surface, com- 
pletely surrounded by a flood of lava. Dana has given an instructive 
account of the descent of a lava-stream from Kilauea in June 1840. 
_Islet-like spaces of forest were left in the midst of the lava, many of 
the trees being still alive. Where the lava flowed round the trees 
the stumps were usually consumed, and cylindrical holes or casts 
remained in the lava, either empty or filled with charcoal. In many 
eases the fallen crown of the tree lay near, and so little damaged that 
the epiphytic plants on it began to grow again. Yet so fluid was 
the lava that it hung in pendent stalactites from the branches, 
which nevertheless, though clasped round by the molten rock, had 
barely their bark scorched. Again, for nearly 100 years there has 
lain on the flank of Etna a large sheet of ice, which, originally in 
the form of a thick mass of snow, was overflowed by lava and has 
thereby been protected from the evaporation and thaw which would 
_ certainly have dissipated it long ago, had it been exposed to the air. 
The heat of the lava has not sufficed to melt it. In other cases snow 
and ice have been melted in large quantities by overflowing lava. 
