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. Brooks f 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 Pluronian. 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 yournal of Science, iii, vol. xi, p. iig. 

 \ American Journal of Science, iii, March, 1876. 

 VOL. IV. 29. 



