THE LOWER HURONIAN. 371 



would enable one to determine their similarity in chemical composition to 

 the adjacent granites. The diiferences between them, which are textural, 

 can be explained by the fact that these offshoots occur in so much smaller 

 quantity that naturally they would not acquire the same textures as the 

 coarser-grained granites and granite-porphyries of the large massives. 



It has already been stated that these acid rocks occur as dikes in the 

 adjacent formations. In some instances their dike character is not clearly 

 shown, actual contacts between them and the rocks occurring nearest to 

 them, and tlii'ough which they cut, being wanting. From the fact that 

 they are igneous rocks and more or less completely surrounded by other 

 erujjtives or by sedimentaries, they are supposed to be intrusive in these. 

 They have been found cutting all of the rocks thus far described from the 

 Vermilion district, those of eruptive as well as those of sedimentary origin, 

 with the exception of the various Lower Huronian granites. They are con- 

 sequently known to be younger than all of the rocks which they cut. Their 

 relations to the Upper Huronian sediments and to the great Dulutli gabbro 

 of Keweenawan age have not been determined conclusively. However, 

 as the result of a reconnaissance in the Keweenawan of the Lake Superior 

 region, it has been found that the Keweenawan is cut by acid rocks. 

 While these hav^ not been connected petrographically with the dikes in the 

 Vermilion district, nevertheless it may be well to suggest the possibility 

 that at least some of the acid dikes in the Vermilion correspond to those 

 which cut through the Keweenawan rocks. 



BASIC AND INTBR3IEDIATE IJS^TRUSIVBS OF LOWER HURONIAK AGE. 



At various places in the Vermilion district basic and intermediate 

 dikes have been observed cutting the country rock. These can be easily 

 divided into dikes of apparently different age by using as a criterion for 

 this the difference in alteration. This macroscopically determined difference 

 is substantiated by the condition of the rocks as shown by microscopic 

 examination. On the one hand there are certain dikes which are composed 

 of very fresh dolerite and basalt and^which show distinct selvages. These 

 cut through all the other rocks of the Vermilion district, including the 

 gabbro. Just outside of the district are dikes identical in character with 

 these and cutting even the acid i-ed rock of the Keweenawan, which itself is 

 known to cut the gabbro. These fresh dikes are clearly of Keweenawan 



