4 Prof. J. W. Judd—The Lavas of Krakatoa. 
in one hundred parts by weight. 
ae 91:0 
Glassireseccusereccencenas 90-0 
Helspars’: iassasecenscoteees 6°5 wi ( 6:0 ) 
IDENT: Gooeoaseabodae0005000 1:4 aoc ( 1°36) 
AUSILC oo cednacsbadsemeaceenl 0°6 sts ( 0°64) 
Magnetite .........eccececee 15 wae ( 1:00) 
The estimate made by Retgers, which I have placed in brackets 
beside my own, was based on the study of a dust which fell at 
Buitenzorg. But this material cannot exactly represent the original 
rock, seeing that during the passage of the dust through the air, the 
magnetite and other heavy minerals tend to fall sooner than the 
glass, while, on the other hand, excessively fine particles of the 
latter are left behind in the atmosphere. Hence, the dust which 
falls at any particular point cannot possibly be taken as a fair 
sample of the whole mass of the rock. 
The Krakatoa-rocks, it will be seen, consist of two very distinct 
materials, a crystalline portion making up one-tenth of the whole 
mass, and a colloid or glassy portion which forms no less than 
nine-tenths of the whole. The chemical composition of the whole 
rock and of these two constituents is as follows :— 
Composition of Composition of Glass Composition of Crystals 
rock. forming 90 °/, of forming 10 °/, of 
rock, rock, 
Silica with Titanic acid. 70:0 ves 72°8 Aa 48°7 
JNU | jAonebabesonssoede 15:0 igs 14°7 19°3 
Oxides of Iron ......... 4:0 om 1°8 18-0 
Time asian Ue Behe 377 374 6°9 
Magnesia .......0.00c8s000s 1°3 1:0 2°5 
Sod anes aiencaee 4-1 4-1 3°8 
Potash penuh ic sat ceceent 1-9 BB 2:2 0°8 
crystalline and a glassy portion, and these have nearly the same com- 
position as those two portions of the Krakatoa-rock. All the rocks 
contain the same minerals, felspar, enstatite, augite, and magnetite ; 
the analyses of which yield very similar results. In all of them, 
too, the relative proportions of these several minerals do not appear 
to be very different from that we have found to exist in the Krakatoa 
rocks. I have already shown that the composition of the glass in 
these different rocks is not very dissimilar. Nevertheless the 
ultimate chemical composition of these various rocks, all of which 
are called by petrographers by the same name of “ enstatite-andesite,” 
or “hypersthene-andesite,” is found to differ in the most remarkable 
manner. 
This fact is illustrated by the following table :— 
Rock of Santorin Buffalo Cheviot — Recent lavas Rocks of 
dykes. Peaks lava, rocks. of Santorin, Krakatoa. 
Silicameeeeneeeee OMS Ow es ee BBB... dooced GEC) boos Glas, Aces 70-0 
Alumina ...... 20°1 Bamecs il OS 2 Re inne PGES oeeeer TAS canine 15-0 
Oxides of Iron. 11°6 ...... OB whe Dali ue ae Gd: Mean 4:0 
Time ees eases DO tte FS ee ANAL VEN Sel ee 37 
Magnesia ...... el! Teassaa BIG Sadana 408” oopetio ILS). gonddo 1:3 
Soda wgner ADS] ie eed Owls 5 eae Gil Stee ein Ce aan cO 4:1 
IRotasht ree seeee OREM eeee ey tee 308 Uh aa e PION Ceoooe 1:9 
At one end of this series we have a rock which is basic in its 
