248 THE VERMILION IRON-BEARING DISTRICT. 



find sedimentary rocks — conglomerates and g-raywackes — which have been 

 derived from them. In many cases, moreover, it is only after very careful 

 and patient examination that the igneous rocks can be separated from the 

 derived sedimentaries, for where the constituents of these derivative rocks 

 have been merely cemented together without having been much rolled 

 and rounded, and where the deposits are not well stratified, the resemblance 

 between the true igneous rocks and the rocks derived from them is very 

 great indeed. 



The igneous rocks that occur at a considerable distance from the lake 

 are exposed over only very small areas. In some cases the exposure is suf- 

 ficient to show that the rocks are dikes in older rocks, and very commonly 

 this relation is inferred from the occurrence of these rocks in the midst of 

 numerous exposures of rocks belonging to formations which, from facts 

 observed in other localities, are known to be of greater age than the 

 eruptives. 



Topography. — The eruptives usually occupy the crests of hills, or occur 

 in rounded or oval hills higher than those occupied by the surrounding 

 rocks. They thus are seen to influence the topography to a considerable 

 extent. This influence is, of course, best shown in those areas where the 

 rocks occur in large quantity, as, for example, the islands in Vermilion 

 Lake and the lake shores, rather than at places some distance away, where 

 they occur as very small masses. 



PETROGRAPHIC CHARACTERS. 



The rocks considered in this section form a complex varying in both 

 macroscopic and microscopic characters, as well as in chemical composition, 

 yet in spite of these variations their close field relations and their char- 

 acters as determined by laboratory study show that they all belong to one 

 petrographic province and that they were formed at the same geologic 

 period. 



All of the rocks belonging to this series of eruptives are very light 

 colored, at the most showing slaty-gray to greenish-gray colors upon fresh 

 fracture. On weathered surfaces they are usually white or light gray, 

 varying to yellowish or pinkish. 



Macroscopic characters. — The rocks under discussion vary from fine- 

 grained granites to those of coarse grain, and from porphyries with felsitic 

 groundmass and rare phenocrysts to coarse-grained granite-porphyries. 



