Prof. J. W. Judd—The Lavas of Krakatoa. fh 
In all these cases it is clear that the rocks contain a considerable 
proportion of volatile materials, which are given off at a high 
temperature. 
When we come to study the several varieties of the lava of 
Krakatoa in the field, we are struck by remarkable differences in the 
mode in which they must have behaved while in a liquid condition. 
The stony lava and the pitchstone it is seen were extruded in massive 
lava-streams with an almost total absence of pumice or scoriz; the 
obsidian, on the other hand, was almost wholly distended into a 
pumice, throughout which the porphyritic crystals of the rock are 
found entaneled. It is evident that every part of the glass has been 
impregnated with the volatile constituents, which by their escape 
have caused its conversion into pumice, for the whole of the glass is 
drawn out into the finest rods and plates; indeed, it is only in the 
case of small fragments which have been suddenly cooled before the 
volatile ingredients have been permitted to escape, that we are able 
to judge of the characters of the glass from which the pumice was 
formed By cutting blocks of pumice of definite size, and comparing 
their weight with that of masses of glass of the same size (this 
being easily determined when the specific gravity of the glass is 
known), I have been able to calculate the amount of distension 
which has taken place in the glass during its conversion into pumice. 
The more compact varieties of pumice have 34 times the bulk of the 
glass from which they were formed; in the more common looser 
varieties the distension has been at least 5 54 times the original ; and 
in some varieties the bulk of the air- cavities is 7, 8 or even 9 times 
that of the enclosing glass. 
The stony lavas and the pitchstones differ mainly in the amount 
of devitrification which the glassy base has undergone. And this 
we have every reason to believe was determined by the rate at which 
cooling took place. We find indeed every g gradation between the 
one type and the other, just as we do in the Cheviot Hills; at 
Steerhope, near Yetholm, in that district, a great lava-stream may 
be seen, the central portion of which consists of a stony rock, while 
the superficial layers graduate into well-known pitchstone. There 
can be little doubt that, in the same way at Krakatoa, the more 
rapidly cooled portions of the stony lava have formed the porphy- 
ritic pitchstone. 
But the obsidian of Krakatoa presents many very striking 
differences from the other two kinds of rock; and for some of these 
the more easy fusibility of its base affords a ready explanation. 
Thus, little knots of the pitchstone are often found scattered through 
the obsidian; and in the pumice formed from the obsidian these 
stand out in relief upon the abraded surfaces, in the same way as do 
the porphyritic crystals). Now when such knots of pitchstone are 
examined microscopically, the felspars in them are found to have 
undergone a most wonderful amount of corrosion; often the larger 
part of their bulk has been redissolved in the heated magma in 
which they have been enveloped, they are completely honeycombed, 
and sometimes reduced to mere skeletons. 
