6 Prof. J. W. Judd—The Lavas of Krakatoa. 
mass of the rocks, the crystalline or porphyritic portions being the 
same in all three types.* 
In most of the older ejections the base is of a dull reddish-grey 
tint, and stony texture. Under the microscope the glass of the base 
is seen to have undergone a great amount of devitrification, and to 
be filled by a mesh of microlites of felspar, pyroxene, hornblende 
and iron-oxides, between which minute portions of the glassy base 
are left. Frequently this base is cavernous, and in such cases the 
cavities are lined with beautiful crystallizations of quartz, tridymite 
and hornblende. These are probably the results of secondary 
alteration. The fact of water having percolated through the rock 
is shown by the more or less complete conversion of the magnetite 
into hydrated ferric oxide.: 
Associated with this porphyritic stony lava, we find a second type 
of rock, the base of which is of a jet-black colour and resinous 
lustre. This beautiful velvety black rock is an admirable example 
of a porphyritic pitchstone, and strikingly resembles the rock of the 
Cheviot Hills and some of the Santorin lavas. The base contains 
fewer microlites, but is crowded with crystallites, both belonites and 
trichites, and these not unfrequently, according to Retgers, form the 
variety of spherulites called by Rosenbusch “ felso-spherites.” The 
rock, moreover, often exhibits a fluidal structure. 
In the third type of rock occurring at Krakatoa, we find the 
porphyritic crystals still the same, but embedded in an almost 
perfect glass, which is of a rich amber-brown colour when seen in 
large fragments, but almost colourless in thin sections by transmitted 
light. This rock has been called a porphyritic obsidian. Under the 
microscope, the glassy base of this rock is found to contain only a 
very few widely-scattered microlites and crystallites. 
But the most striking difference between the base or ground-mass 
of these three types of lava becomes apparent when they are sub- 
jected to high temperatures. If a fragment of the stony lava or of 
the pitchstone be acted upon by a strong blowpipe-jet, urged by a 
foot-bellows, it may be rendered white-hot without exhibiting signs 
of fusion. But if we treat a fragment of the obsidian in the same 
way, we shall find that on approaching a white heat, it begins to 
melt, and in doing so swells up into a cauliflower-like mass, five or 
six times the size of the original. When this mass is cooled and 
examined, it is found to be a dirty brownish-white pumice, identical 
in all its characters with the material that was thrown out in such 
vast quantities during the last great eruption of Krakatoa. This 
identity in character is seen to be even more marked if we make 
thin sections of the natural and of the artificial products, and 
examine them under the microscope. In its behaviour the obsidian 
of Krakatoa behaves indeed precisely like the Marekanite of Okhotz 
and a glassy rock from Fife which I have previously described.’ 
1 In the Report on ‘‘ Krakatoa’’ by the Royal Society Committee, plates ii. and 
lil. are devoted to the illustration of the characters of the rocks of Krakatoa as 
exhibited in microscopic sections. 
® Got. Mac. Dee. III. Vol. III. (1886), p. 243. Quart. Journ. Geol. Soe. vol. 
xlii. (1886), p. 429. 
