LITHOLOGY. 225 
less quartz. In others a triclinic feldspar becomes prominent, the schist- 
ose character disappears, and a diorite composed of plagioclase and horn- 
blende results. To these three species,—diorite, amphibolite, and chlorite 
schist,—all these greenstones may be referred, but the variations in the 
proportion of ingredients and the presence of accessories diversify the 
rocks to that extent which is to be expected in sedimentary formations. 
They form a group which has excited much interest, and some general 
remarks in reference to them will be made after they have been described. 
As a formation these rocks are widely distributed. Dr. Hunt de- 
scribed their occurrence and geological relationships in his report on the 
geology of Canada, and Prof. Dana* has pointed out their stratigraphical 
relationships in the New Haven region. Mr. T. B. Brookst has found 
them widely distributed over the southern shores of Lake Superior, and 
has described and classified them. Our particular formation was first 
noticed by Prof. E. Hitchcock, who called them all greenstones. Prof. 
Hitchcock, of this survey, has studied our formation with great care, and 
referred the rocks to the Huronian age. Other investigators have deter- 
mined like formations also to belong to the Huronian. In Europe, rocks 
thus associated frequently build formations; but I think that enough 
has been said to indicate that these rocks form a defined group, the 
members of which are very likely to be associated together, and that 
therefore anything that can be deduced from the study of our rocks may 
be quite generally applied. 
METAMORPHIC DIORITE. 
This is essentially a compound of hornblende and a triclinic feldspar, 
and is therefore of the same composition as the rock which has been 
described as basic eruptive. This variety, however, is found interstrat- 
ified with schists of various kinds. It bears at times marks of stratifica- 
tion itself, and has been repeatedly shown to belong to the formation in 
which it occurs, although the existence of diorites other than eruptive 
is not as yet generally admitted. It is not at all surprising that two rocks 
of the same composition should originate in very diverse ways, for among 
* American Fournal of Science, iii, vol. xi, p. 119. 
ft American Fournal of Science, iii, March, 1876. 
VOL, IV. 29. 
