46 



THE CRYSTAL FALLS IRON-BEAEING DISTRICT. 



BASIC DIKES IN THE ARCHEAN. 



The influence of tlie dikes on the character of the topography has 

 ah-eady been uientioued. They occur in long narrow bands of varying 

 widths, and with one exception are markedly schistose. Considering the 

 o-ranite on a large scale as an approximately homogeneous mass, we would 

 expect to hud lines of weakness, which might be indicated by the arrange- 

 ment of the dikes. No such definite arrangement can be seen, however, as 

 the dikes are found to extend in all directions. A good example of their 

 mode of occurrence may be seen in fig. 5, which also illustrates very clearly 

 their influence on the topography. A small valley, in sec. 1, T. 44, R. 32, 

 through which a brook flows, is occupied by the main dike, from which 



diverge the smaller ones, 

 penetrating the granite on 

 both sides. These, not 

 having been much more 

 deeply eroded than the 

 granite, do not form chan- 

 nels deep enough to be 

 shown on a map with a 

 10-foot contour interval. 

 It is without doubt owing 

 to the fact that the dikes 

 weather so much more 

 readily than does the granite that we may partly explain the comparative 

 scarcity of exposures. The depressions separating the granite knobs are 

 believed to indicate in many cases the position of dikes, but being now 

 filled with glacial deposits, the underlying dike rocks, if such are present, 

 are covered. Thus we find them exposed only where erosion has cleared 

 this debris away, or where portions of the dike still border the steep sides 

 of the granite on the sides of depressions. 



The dikes may be classified as (1) earlier dikes, showing a schistose 

 structure, and with no trace of igneous textures, and (2) later massive dikes, 

 showing original igneous textures. 



(1) SCHISTOSE DIKES. 



The general character of these rocks occurring as dikes ma)^ be briefly 

 mentioned. Thej^ are schistose, for the most part fine grained, and black 



Pig. S.— Illustration of the ellect on the topography of the differential erosion 

 of basic dikes and granite. 



