Adams— Contribution to our knowledge of the Laurentian. 65 
holding the quartz of the original rocks as grains scattered 
thickly through it. When sorted by the action of moving 
water it gives rise to beds of sand and clay. The chemical 
processes at work during this process of decay consist in the 
more or less complete removal of the alkalies of the feldspar, 
and of the decomposition of the iron magnesia constituents 
with the loss of a large proportion of the lime as compared 
with the magnesia. On comparing the analyses of a series of 
granites with those of a series of slates, as for instance those 
given in Roth’s “Gesteins Analyzen,” the latter are seen to be 
on an average considerably higher in alumina and much lower 
in alkalies, while at the same time they are lower in silica, 
which has been separated both as sand and in combination 
with the alkalies which have gone into solution, and in most 
cases contain more magnesia than lime instead of more lime 
than magnesia, as is usual in granites. © 
The average percentage of alkalies in the thirty-seven an- 
alyses of granites from various parts of the world given by 
Roth in his work above mentioned is 7°35 per cent, while 
twenty-three primitive clay slates (Urthonschiefer) contain on 
an average only 4°70 per cent and twenty-five slates of Silurian 
age 4:82 per cent of alkalies. The slates thus contain on an 
average about two-thirds of the amount of alkali present in 
the average granite. A typical slate is thus distinctly different 
in chemical composition from an ordinary granite, although 
sediments having an intermediate composition are frequently 
_ produced by the disintegration of granite without complete 
decay giving rise to rocks such as arkose, grauwacke, felds- 
pathic sandstones and so on. 
It seemed probable therefore that some evidence as to the 
origin of these apparently sedimentary gneisses might be 
obtained from a study of their chemical composition. Four 
typical representatives of these gneisses were accordingly 
selected and analyzed, as well as one of the gneisses of class 2 
known to be of igneous origin. These analyses are given in 
the accompanying tables, together with analyses of three slates 
and of a granite, for purposes of comparison. Analyses Nos. 
IJ, V, VIL and VIII were'made for me by Mr. Walter C. 
Adams of this University and analysis No. I was made by Mr. 
‘ Nevil N. Evans, Lecturer in Chemistry in McGill University. 
‘To both gentlemen I desire to acknowledge my great indebted- 
ness. 
I. Gneiss from St. Jean de Matha Province of Quebec. A fine 
grained garnetiferous sillimanite Gneiss containing also 
much quartz and orthoclase. Graphite and pyrite are also 
present, the latter causing gneiss to weather to a very rusty 
color. It occurs in thick bands interstratified with white 
Am. Jour. Sc1.—TsIRD SERIES, VOL. L, No. 295.—JuLy, 1895. 
5 
