of Edinhurgh, Session 1883-84. 
481 
however, that in the deep sea sediments a very large part of these 
vitreous splinters has not been derived from the pulverised ejections 
from a volcano, but from the trituration of floating pumice, of which 
we have given above a striking example. It will be understood that 
it is scarcely possible to trace the difference between volcanic ashes, 
properly so called, and the products resulting from the pulverisation 
of floating pumice which we have just indicated. As in the inco- 
herent products of Krakatoa, so we find spread out on the bottom of 
the sea many more vitreous particles, similar to those we have just 
described, than of true volcanic minerals. This is easily explained, 
however, when we remember how the distribution of volcanic dust 
takes place. 
Let us now point out the minerals which can be determined with 
certainty in the ashes of this great eruption ; and we may at once 
remark that they are the same which we have almost always found 
associated in the deposits with the splinters of glass. In general 
all the crystals are fractured, except those which are still embedded 
in a vitreous layer ; this vitreous coating is often crackled and 
bullous. In the ashes of Krakatoa, however, we have not remarked 
the globules of glass which are often described as glued to the 
minerals of volcanic ashes, nor have we seen the drawn-out vitreous 
filaments resembling Peles hair. The minerals of the Krakatoa 
ashes which are susceptible of a rigorous determination belong to 
plagioclase, augite, rhombic pyroxene, and magnetite.* We shall 
presently see the peculiarity which distinguishes each of these 
species in the ashes. 
Among the most frequent minerals, but poorly represented in 
comparison with the vitreous matter, plagioclase felspar comes first. 
This mineral has about the same dimensions as the vitreous frag- 
ments, and, with the exception of the crystals entirely enclosed in 
the pumice matter, is in the form of debris. Sometimes twins on the 
albite plan can be distinguished, and the results of analysis clearly 
indicate that it is triclinic felspar which should almost exclusively 
be found in this ash. But the most interesting crystals of plagio- 
* Lately the works on these same ashes have made known as accidental 
elements pyrites, apatite, and perhaps biotite (?). It is to be remarked, how- 
ever, that these minerals must be extremely rare in comparison with the 
vitreous matters and mineral species above-mentioned. 
