472 The American Naturalist. [May, 
lated to the latter rock, from which it is believed to have been derived 
by pressure. Around the garnets are rims composed of radiating 
tongues of hypersthene or of hornblende. Green hornblende is present 
in the gneiss in addition to the brown variety, and all the other com- 
ponents of the gabbro are represented in either the fresh or the altered 
condition. ) 
The Dykes of the Thousand Islands.—The granites, gneisses 
and other rocks of the Admiralty Group of the Thousand Islands in 
the St. Lawrence River are cut by numerous dikes of a dark rock. 
These, to the number of thirty, have been studied by Smyth.’ They 
are all normal diabases and olivine diabases. In the latter variety the 
olivine is often surrounded by a reaction rim composed of radiating 
plates of tremolite. The magnetite in many of the rocks of both va- 
rieties is separated from the plagioclase by a rim of biotite. This is 
absent when the mineral is in contact with the other rock components, 
hence it is regarded as a true reaction rim between the iron oxides and 
the feldspar. 
Analcite-Diabases from California.—aA series of dykes, from 
San Luis, Obispo Co., California, are described by Fairbanks® as con- 
sisting of two distinct portions. The main one is dark and fine-grained, 
and the other a hard, light, rock cutting the former in dykes. Both 
possess the same general features in the thin section, but the lighter 
rock possesses them in greater perfection. It consists of lath-shaped 
basic plagioclase, lamellar diallage and analcite. The latter mineral 
occurs as irregular masses in the feldspar, in wedge-shaped pieces be- 
tween the plagioclase, in the form of hexagonal or rounded grains 
partly enclosed within the feldspars, and as the lining of cavities in the 
rock. It is supposed to have been derived from nepheline, as the mass 
analysis of the rock shows it to be very rich in sodium: 
SiO, AlO, Fe,0, FeO CaO MgO K,O NaO H,O Cl Total 
50.55 20.48 2.66 4.02 7.30 4.24 2.97 837 .44 tr— 100.33 
The analcite is changed partly to an aggregate of green fibres, and 
partly to natrolite. In the wedge-shaped areas between the plagio- 
clase the mineral also contains prehnite crystals, and is bordered here 
and there by a doubly refracting substance supposed to be a soda feld- 
spar. These are both believed to be alteration products of the analcite. 
5 Trans. N. Y. Acad. Bei., xili, p. 209. 
6 Bull. Dept. Geol. Univ. Cal., Vol. I, p. 273. 
