1890.] Mineralogy and Petrogrphy. tyi 
Keewatin are of eruptive origin. The Coutchiching series embraces mica- 
schists and other lighter colored schistose rocks between the lowest mem- 
bers of which and the gneisses are also evidences of contact action. 
Among the lighter schists are granulites and sericite-porphyroids. In 
the Laurentian a hornblende-syenite gneiss occurs, and in it several 
pieces of twinned sphene were observed. The two craters Mte. Cimi- 
no and the Lago di Vico in central Italy, though but parts of the same 
great volcano, like Mts. Somma and Vesuvius, have during their dif- 
ferent periods erupted different kinds of lava. The lavas of Cimino 
have an andesitic habit. They are to be classed with the mica and 
augite andesites, the latter of which contain porphyritic sanidine 
and olivine. The younger crater, Lago di Vico, has poured forth 
leucite bearing rocks, of which leucitophyre, leucite-tephrite, leucite- 
basanite and phonolites are the prevalent types. A leucite-trachyte, 
placed by Rosenbusch among the phonolites, is the latest lava of 
the older crater. It is an intermediate type between the predomi- 
nant lavas of Cimino and those of the later Lago di Vico. Besides the 
lavas, the former crater cast forth sanidnite bombs and calc-silicate 
bombs, containing garnets and vesuvianite. In a brochure on the 
Obere Weilerthal, E. Cohen ‘ gives an interesting account of the erup- 
tive and sedimentary rocks occurring in the Weilerthal south of the 
rocks made famous by Rosenbusch under the name Steigerschiefer. 
Those described by Cohen are granite, gneiss, quartzite-schists, 
phyllites, granite, porphyry, augite porphyry and minettes. t 
granite is a brown hornblende in prismatic crystals. Their specific 
gravity varies between 3.082 and 3.140, and their composition is as 
follows : 


SiO, ALO, FeO, FeO CaO MgO Na,O H,O 
Darker var. 51.36 4.14 2.17 10.04 II.9I 17-14 1.86 1.38 
Lighter var. 51.82 4.17 2.34 9.84 12.18 9.18 2.44 1.93 
Many of the other rocks described present interesting features, but 
none of great petrographical importance, An examination 5 of the 
northern slopes of Cader Idris, Merionethshire, Wales, discloses inter- 
bedded slates, tuffs, and massive eruptive rocks of considerable interest. 
Among the sedimentary rocks is the well-known pisolitic ironstone, in 
which the pisolitic structure is now represented by magnetite crystals 
in a cement of green iron silicate. This structure was originally pro- 

3 W. Deecke, Neues Jahrb. fir Min., B. B. VI., 1889, p- 205. 
4 Abh. zur Geol. Specialk. v. Elsass-Lothringen, B. III., H. III., p. 137- 
5 Quar. Journ. Geol. Soc., August, 1889, p. 432. 
