Prof. J. W. Judd—The Lavas of Krakatoa. 5) 
That there is a very great similarity in the composition of the 
glassy basis of these various rocks is shown by the following 
analyses :— 
Glass of Santorin Glass of Cheviot rocks Glass of Krakatoa 
lava (Fouque). (Ebers in Petersen). ee 
Silica with Titanic acid. 71:5 ae “lei Bee 72°8  (68°8) 
/ Alling), egg epaeoeeanseonen 16°8 14:6 14:7 = (15°9) 
Oxidestoi Miron! 5.255... 0'8 3°3 IS 5 (CGPI) 
ITN |, ERasscaceae eereecee 0°8 2°9 Meee NC iS) 
WIP ERTS GanngpeesonepSEbop 0-7 0°3 NO), {( il9%4)) 
SOUR), |. Gochabe sae eesanenede 74 S6E 2°4 Alors 3 {( ePil)) 
HO UASIME eae ct cs aseeteaees 2°0 5:4 Bo {Csieil)) 
The statement of these and subsequent analyses has been cal- 
culated independently of water and “loss on ignition,” not that I 
regard these volatile materials as being unimportant, but because the 
methods adopted by different chemists for their determination are so 
various, that their inclusion would make the comparisons less satis- 
factory. The analysis of the Krakatoa-glass is calculated from the 
bulk analysis of the pumice, compared with that of the several 
crystalline constituents, the proportion of each of these being known. 
I have placed in brackets besides this calculated analysis the actual 
analysis made by Retgers; but concerning this last it must be 
remarked that it is the analysis of the glass of the dust which fell 
nearly a hundred miles away from the volcano, from which some 
of the lighter particles had been winnowed out in its passage through 
the atmosphere. Moreover, it was found impossible to separate the 
finer grains of magnetite from this glass, so that, as admitted by its 
author, the silica is too low, and the iron-oxides too high. ‘The 
same remark probably applies, though perhaps in a less degree, to 
the analysis of the glass of the Cheviot-rocks. 
A very obvious defect in our modern methods of petrographic 
nomenclature and classification is that they are based on a qualitative 
and not on a quantitative determination of the mineralogical consti- 
tution of the rocks. When the relative proportions of the mineral 
constituents of a rock are taken into consideration, as well as their 
nature, we are led to some very interesting considerations. 
At present, so far as 1 am aware, we have no other simple method 
available for determining the relative proportions of the constituents 
of a rock than the one suggested by Sorby, that of drawing the 
outlines of crystals seen in section under the microscope, and then 
cutting out and weighing on a delicate balance the portions of paper 
representing each of the minerals. By obtaining a photograph 
instead of a drawing of the section, as was suswested by the late 
Sir Richard Daintree, one source of error in such determinations 
may perhaps be removed. Of course, if we have a bulk-analysis of 
the rock, and analyses of each of its mineral constituents, it is 
possible to calculate the proportions which these latter bear to one 
another. 
By a series of determinations of this kind, checked in a great 
number of different ways, I have convinced myself that the consti- 
tution of the Krakatoa-rock may be represented as follows :— 
