222 T. S. Hunt on the Chemical and Mineralogical 
ratio between the alumina and the alkali in the feldspar just 
named is 3:1, it becomes 6:1 in margarodite, an : 1 in mus- 
covite. The appearance of these micas in a rock, then, denotes a 
diminution in the amount of alkali, until in some strata the feld- 
spar almost entirely disappears, and the rock becomes a quartz- 
ose mica schist, In sediments still farther deprived of alkali, 
metamorphism gives rise to schists filled with crystals of kyanite, 
or andalusite, simple silicates of alumina, into whose composi- 
tion alkalies do not enter; or, in case the sediment still retains 
oxyd of iron, staurotide and iron-alumina garnet take their 
formed when magnesia and iron replace lime. In all these cases, 
the excess of the silicates of earthy protoxyds over the silicate 
of alumina is represented in the altered strata by hornblende, py- 
roxene, olivine, and similar species; which give rise by their 
admixture with the double aluminous silicates, to diorite, dole- 
rite, diabase, euphotide, eclogite, and similar compound rocks. 
n eastern North America, the crystalline strata, so far as yet 
studied, may be conveniently classed in five groups, correspond- 
ing to as many different geological series, four of which will be 
considered in the present paper. 
1. The Laurentian system represents the oldest known rocks 
of the globe, and is supposed to be the equivalent of the Pnm 
itive Gneiss formation of Scandinavia, and that of the Western 
Islands of Scotland to which also the name of Laurentian is DoW 
applied. It has been investigated in Canada along a continuous 
outcrop from the coast of Labrador to Lake Superior, and also 
over a considerable area in Northern New York. ee 
_ 2. Associated with this system is a series of strata charactel 
ized by a great development of anorthosites, of which the hy- 
persthenite, or opalescent feldspar-rock of Labrador, may be taxe® 
asatype. These strata overlie the Laurentian gneiss, and @ 
