THE 
GEOLOGICAL MAGAZINE. 
NEW “SERIESS DECADE lll *VOE. Vv: 
No. I—JANUARY, 1888. 
Gi GaN PAG, PASE Gin aaSe 
——>—__ 
T.—Tue Natura History or Lavas as ILLUSTRATED BY 
THE MaTERIALS EJECTED FROM KRAKATOA.! 
By Prof. Joun W. Jupp, F.R.S., Pres.G.S. 
HORTLY after the great volcanic eruption at Krakatoa, in 
August, 1885, the Royal Society appointed a Committee to 
collect and sift the records of that remarkable outbreak and to study 
the effects which appear to have resulted from it. The investigation 
of the voleanic phenomena and of the materials ejected from the 
volcano having been committed to me, I have been led to some 
conclusions of considerable geological interest, the full discussion of 
which could scarcely find a place in the report which has been pub- 
lished by the Committee. It is these general results which I propose 
to discuss in the following communication. 
For the opportunity of studying the different rocks of Krakatoa, 
I am indebted to many correspondents who have supplied me with 
the necessary materials, and especially to M. René Bréon, who visited 
the volcano shortly after the eruption. 
The central crater of Krakatoa has always given vent to lavas of 
the same general type—namely pyroxene-andesites, or rather, as 
they should perhaps be called, dacites ; though from a lateral vent 
a great mass of basaltic lavas and tuffs were ejected during one 
epoch in the past history of the volcano. 
It is the pyroxene-andesites to which I especially desire to call 
attention ; for their study, in connection with that of other rocks of 
the same class, suggests a number of very interesting generalizations. 
The pyroxenes contained in these rocks belong to two species. The 
most abundant is one crystallizing in forms belonging to the ortho- 
rhombic system, in the composition of which the base magnesia 
predominates over the lime; it is a ferriferous enstatite or hypersthene. 
The less frequently occurring pyroxene is one crystallizing in forms 
of the clinorhombic system, in which lime preponderates over the 
magnesia: it is a true augite. Not only do these two forms of 
pyroxene always occur together, but they are sometimes found inter- 
grown with their corresponding crystallographic planes in parallel 
positions. From the predominating pyroxene in these rocks they 
1 A paper read at the Meeting of the British Association in Manchester (September, 
1887). 
DECADE II.—VyOL. V.—NO. I. 1 
