Mineralogy and Petrography. 167 
MINERALOGY AND PETROGRAPHY:! 
PETROGRAPHICAL News.—As the result of a recent trip 
through the southern extremity of Africa, E. Cohen? has succeeded 
in giving us quite a good deal of information regarding the Palæo- 
zoic formations of the Cape States. The pre-Devonian schists of 
the coast region have been treated in another place.* In the pre- 
sent paper the author confines himself to the various members of 
the Devonian and Carboniferous systems, and other formations 
overlying these. The most widespread rocks in this region are 
sandstones, graywackes and conglomerates. e Karroo formation 
(Triassic) Cohen divides into a lower, a middle and an upper series. 
The lower series comprises fragmental rocks with an occasional in- 
tercalated layer of an eruptive. The middle series is characterized 
by the number of layers of eruptives intruded between those of 
sedimentary rocks as well as by the number of dykes cutting across 
the latter. The eruptives, with a single exception, are plagioclase 
augite rocks. By far the larger proportion of these belong to the 
diabase family, many of them being olivine bearing. In the latter 
the peecilitic structure is frequently well marked. The diabases, 
quartz diabases, proterobase and diabase porphyrites, of both the 
intercalated layers and the dykes, are regarded by Cohen (as the 
result of careful analyses) as mere phases of the same magma. 
The single exception to the prevailing plagioclase-augite eruptives 
mentioned above is in the case of a dyke-cutting olivine diabase. 
The material of this bears a strong resemblance to mica syenite. 
At the points where the diabase layers come in contact with the 
interstratified sandstone beds the latter have been subjected to con- 
siderable alteration. The unaltered rock is an ochre-yellow, fine 
la sandstone, made up of quartz and colorless mica, besides a 
ittle iron hydroxide and earthy material. As it approaches the 
diabase it gradiially loses all traces of its bedding planes, and in it 
is developed a green chloritic mineral, whose nature was not deter- 
mined. Nearer to the eruptive the chloritic mineral increases in 
quantity, and in addition there is a development of biotite and _ 
a disappearance of the earthy material, which has probably gone to 
make up the biotite. In immediate contact with the diabase the 
sandstone has been entirely changed to a typical black hornfels. 
In it all the constituents have taken on a concretionary form. 
Analyses of the unaltered sandstone and of two typical altered 
phases teach that the change in the nature of the sedimentary rock 
is not due to any addition of diabase material. The dyke rocks 
produce but little alteration in the neighboring fragmentals. In 
1 Edited by Dr. W. S. Bayley, Madison, Wisconsin. 
s Neues Jahrb. f. Min., etc., 1887, Beil. Bd. v. p. 195. 
Ib., 1874, p. 460. 
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