1891.] Mineralogy and Petrography. 571 
and depression, and probably of volcanic activity. He thinks also 
that the tendency of the evidence is to show that some granitoid 
rocks, such as those classed in Wales under the name Dimetian, are 
among the very oldest of the pre-Cambrian rocks which are now found 
exposed, and that some quartzites, porcellanites, and schists occupy 
an intermediate position in point of age between these granitoid rocks 
and the Pebidian series. (Geol. Mag., Nov., 1890.) 
MINERALOGY AND PETROGRAPHY.' 
Petrographical News.—Mount Avidlo, in the southern Alps, 
consists in part of tonalite and in part of a quartz-mica-diorite, both 
of which intersect a series of crystalline schists, in which contact 
alteration has been effected. The tonalite is the rock so well known 
as comprising a large part. of the Adamello group of the Alps. It is 
essentially a hornblendic quartz-mica-diorite. A garnetiferous variety 
‘is described by Salomon? as an endomorphous contact product. It is 
characterized by the possession of plagioclase zonally developed, with 
the most acid zones on the exterior. The extinction of crystalsevaries 
as much as 30°, being by this much greater in the nucleus than in the 
peripheral portions. The quartz-mica-diorite forms a boss only two 
kilometers distant from that of the tonalite, but it is regarded by the 
author as having no genetical relation with the latter. These two 
masses of eruptives are surrounded by two series of schists: a younger 
series including phyllites and epidote-amphibolites, and an older one 
embracing gneiss and mica-schists. The former are in contact with 
the diorite, by which they have been changed into rocks composed 
essentially of quartz, muscovite, biotite, chlorite, and andalusite, of 
which the biotite and andalusite are new products. Corundum, tour- 
maline, sillimanite, and zircon are also new products, but are — 
only in small quantity. A cordierite-biotite rock, consisting of 
was found as an inclusion 1n 
the diorite. According to the degree of alteration effected in them 
the rocks are separated into two zones: an outer one, the zone of the 
ilmenite-frucht-schiefer, in which the phyllites have suffered merely ls 
change of their chlorite into biotite, and an inner zone, 1 which 
andalusite is an important constituent. The schists around the tonalite 
belong to’ the older series of gneiss and mica-schists. These have been 
1 Edited by Dr. W. S. Bayley, Colby University, Waterville, Me. 
? Zeits. d. deutsch. geol. Ges., XLIL., 1899, p. 450- 
