370 THE VERMILION IRON-BEARING DISTRICT. 



warranted by the occurrence of these other rocks which surround them in 

 numerous exposures. They are of such small areal distribution that they 

 do not materially influence the topography in general, although, since they 

 are generally harder than the rocks through which they cut, they do 

 exert a local influence and are found forming knolls or ridges. 



PETROGRAPHIC CHARACTERS 



Macroscopic. — These dikes vary in grain from very fine-grained, almost 

 cryptocrystalline rocks, to those which may be classed as coarse-grained 

 ones. Moreover, a porphyritic structure is of very common occurrence, the 

 quarts being sometimes the sole phenocryst though at other times both 

 quartz and feldspar occur as phenocrysts. In color there is likewise con- 

 siderable variation. For the most part the rocks are gray to pink on fresh 

 fracture. Some of the finer-grained ones, however, are dark purplish. The 

 weathered surfaces are nearly always pink to reddish. The dikes vary 

 greatly in width. They are usually narrow, but some dikes as much as 10 

 feet wide have been observed, and the grain usually varies with the width, 

 the finer-grained rocks occurring in the narrow^- dikes and as the selvage 

 of the wider ones. 



Microscopic. — Under the microscope one can recognize phenocrysts of 

 green hornblende in association with phenocrysts of feldspar and quartz- 

 The quartz is relatively scarce. These jDhenocrj^sts lie in an exceedingly 

 fine-grained groundmass with, in some cases, grains too small to permit 

 their characters to be recognized. Generally it can be seen that the ground- 

 mass is made up of quartz and feldspar, flakes of chlorite and sericite, and 

 some iron ore. Occasionally the grains and flakes are arranged in parallel 

 lines which so bend around the phenocrysts as to bring out a very well- 

 marked flowage texture. All of the minerals have their usual characters, 

 and hence no description will be given of them. 



RELATIONS TO ADJACENT FORMATIONS. 



It is believed that these acid dikes are offshoots from the various granites 

 described in this chapter. They show a general petrographic similarity to 

 them. Still they are not so much like them as to warrant one in making a 

 positive statement that they are the same. Moreover, they have not been 

 connected in the field, nor have any chemical analyses been made which 



