1050 The American Naturalist. [December, 
PETROGRAPHY:? 
Petrography of the Marquette Iron Range.—Van Hise and 
Bayley’ in their recent description of the Marquette Iron Range in 
Michigan, give accounts of the petrography of the Archean and of the 
Algonkian rocks occurring in the region. The pre-Algonkian rocks 
comprise granite-gneisses, syenites, hornblendic and micaceous schists, 
and a series of green schists that are cut by peridotites, and by acid 
and basic dykes. The granites or gneisses are notable for the large 
quantity of microcline they contain. Although this is present in the 
most massive phases of the granite, it is much more abundant in those 
phases that are schistose as the result of pressure. These rocks are 
much crushed, and have developed in them great quantities of new 
plagioclase, quartz and muscovite in addition to the microcline above 
referred to. The syenites are mainly granitic aggregates of orthoclase 
and hornblende, with some plagioclase and a number of secondary pro- 
ducts formed under the influence of pressure. These are the same in 
character as the secondary products formed in the granite under simi- 
lar conditions. The green schists are squeezed surface materials. 
Some of them are squeezed tuffs and others squeezed lavas. The for- 
mer contain numerous pebble-like masses that are taken to be bombs 
and rounded lava fragments. The present constituents of the schists 
are chlorite, sericite, calcite, plagioclase, quartz and sometimes epidote. 
Much of the plagioclase is in broken crystals lying in a matrix formed 
of smaller fragments of the same mineral cemented together by a felt 
of the others in a very finely crystallized groundmass. The structure 
of many of the schists is typically tuffaceous. These rocks indicate 
clearly the existence of volcanoes in pre-Algonkian time. The basic . 
schists are associated with acid ones that seem to be squeezed rhyolitic 
lavas. Analyses of these two classes of schists follow : 
SiO, TiO, AlOs FeO; FeO CaO MgO K,O Na:0 P,O; CO: H,0 Total 
Basie schist 61.35 .26 94 4.20 3.46 3,121.05 5.24 .18 1.98 2.61 = 100.84 
Acid schist 70.76 .33 gre 1.46 3.09 .86 1.99 3.50 .47 .26 2.79 =, 99.84 
The basic rocks cutting the gneisses and schists are diabases in 
various stages of alteration, and the acid ones are quartz porphyries. 
These have been carefully described by Williams.’ The peridotite of 
1 Edited by Dr. W. S. Bayley, Colby University, Waterville, Maine. 
? Monograph XXVIII U. S. Geol. Survey, Washington, 1897. 
