* 
1890.] Mineralogy and Petrography. 1071 
superinduced by pressure, with the aid of heat sufficient for the fusion 
of the original sediments. Attention is called to the fact that these 
schists are not members of the great complex underlying the earliest 
sedimentary rocks, but are contemporaneous with some of the latter 
which are probably of Huronian age. After a very thorough dis- 
cussion of fifty-three analyses of plutonic and effusive rocks Rosen- 
busch‘ concludes that the cause of the great variety in the rocks 
extruded from an eruptive center is the capacity of an original magma 
for separating into portions with different compositions (Spaltungs- 
fahigkeit). These different portions may exist under the earth’s crust 
in positions very near each other. From the very nature of the dis- 
cussion, depending as it does upon so much detail, it is impossible to 
reproduce its argument in these pages. It must satisfy our present 
purposes to state that Prof. Rosenbusch thinks the original magma had 
a composition near that of a mixture of elzolite-syenite, peridotite and 
residues of the formulas (NaK)AISi, and R,Si. The residue 
(NaK)AISi, possesses the capacity of taking up silica and yielding 
granitemagma. The first splitting of the original magma yielded deriv- 
ative magmas (theilmagmen), which have solidified as plutonic rocks, 
Further differentiation produced the materials whose solidification 
yielded the effusive rocks. This explanation of the differences existing 
in the composition of the plutonic rocks and their corresponding 
effusives is thought by Rosenbusch to be better than that which ascribes 
them to a separation of the original magma according to the density 
of its parts, whereby the highest portions (those producing the effusive 
tocks) had of necessity a different composition from the lower por- 
tions. The paper contains significant utterances with respect to the 
relations between the geological age of a rock and its structure. It is 
said that the difference between older and younger effusive rocks is 
“that the former have existed on the surface for a longer time than the -7 
latter, and consequently have suffered a series of changes (umbilden- 
den Processen) . . . One needs no great gift of penetration to prophesy 
that in the near future this separation [of the paleovolcanic from the 
neovolcanic rocks] will lack recognition."—— Dahms * has examined 
a set of hand-specimens brought from the Transvaal, Africa, among 
which he recognizes gabros containing pleochroic diallage and augite, 
the two minerals occurring in different parts of the same mass, and 
secondary quartz and hornblende. He finds also diabases and quartz- 
diabases, a quartz-porphyry in whose quartz-phenocrysts are inclusions 
t Miner. u. Petrog. Mitth., X1., 1890, p. 144- 
5 Neues Jahrb. f. Min., etc., Beil. Bd., VII., 1890, p. 90- 
