1895.] Petrography. 471 
South African Republic, has appeared under Molengraaf’s’ name. The 
major portion of the paper is taken up with descriptions of the geologi- 
cal features of the region. There are in it, however, several items of 
petrographic interest. The oldest formation of the region embraces 
granites and crystalline schists. The former rock-type includes tonal- 
ites and orthoclase-plagioclase-microcline granites. In some places 
the rocks show evidences of dynamic metamorphism. Among the rocks 
associated with the granite are sericite-schists, actinolite-schists and 
amphibolites. Above these is another schist formation, comprising 
quartzites, clay slates, corundum-schists and porphyroids, and chiasto- 
lite-schists, cut by diabase dykes. The corundum porphyroid resem- 
bles a feldspar porphry. Large crystals of biotite and large corundum 
individuals are in a groundmass of quartz and chlorite. The whole 
rock is besprinkled with quartz grains. Above the schists are bedded 
fragmentals, with which are associated diabases, quartz-porphyres and 
-amygdaloids. In one of the diabases a diallagic augite and a primary 
hornblende were detected. In the carboniferous sediments south of 
Reitzburg are quartz gabbro and quartz diabases, and in the Rhenos- 
terkop in the diamond fields at Driekop, in the Orange Free States, is 
a quartz-amphibole gabbro containing magnetite, biotite, primary 
hornblende, diallage and plagioclase. The pyroxene is striated paral- 
lell to oP, and is twinned parallel to œ Px. 
The Gabbro of the Adirondacks.—The gabbro associated with 
anorthosites of the Adirondacks are described by Smyth‘ as very simi- 
lar to the Baltimore gabbros. They are best developed at Morehouse- 
ville and at Wilmurt Lake in the valley of West Canada Creek. The 
rock is a norite, in some phases a hypersthene-gabbro, both containing 
a brown hornblende regarded as original. The hypersthene, especially 
in the foliated varieties of the gabbro, which have been rendered 
schistose by pressure, sends tongues out into the contiguous feldspar. 
This stringing out of the pyroxene is so closely connected with the de- 
velopment of the foliation of the rock that it is believed to be a dy- 
namic phenomenon. An analysis of the gabbro gave: 
SiO, .Al,O, Fe,OQ, FeQ MgO CaO NaO K.O H,O Total 
46.85 18.00 616 8.76 843 10.17 219 .09 .80 = 100.95 
A black garnetiferous hornblende gneiss, which is associated with the 
‘other gneisses in the neighborhood of the gabbro, is thought to be re- 
3 Neues Jahrb. f. Miner. B. B., ix, p. 174. 
‘Amer. Jour. Sci., xlviii, 1894, p. 54. 
