344 THE VERMILION IRON-BEARING DISTRICT. 



tlie sediments are, as above stated, granular and in character resemble much 

 more closely the gabbro than they do the sediments proper. At some 

 distance away from the gabbro contact rounded masses of dense greenish 

 and gray rock begin to appear, suiTOunded by the granular contact product. 

 This gives a conglomeratic appearance to the exposure. A little farther 

 away from the gabbro, on a vertical exposure at the lake shore, these 

 pebble-like areas were seen to be aligned, and eventually to pass into 

 parallel unbroken bands. The explanation of this occm-rence is that the 

 gabbro in contact with the sediments caused them to be altered to the 

 peculiar granular contact product. The alteration naturally was most 

 effective nearest the gabbro, and gradually sprea.d, following along the 

 cracks in the rocks, the vei'tical as well as the horizontal cracks. Near the 

 gabbro all of the sediments were changed. Farther away the blocks or 

 fragments of sediments were changed on the exterior most completely. 

 Sometimes this change was so far reaching as to convert into the granular 

 contact product most of the sedimentary block, and to leave only a small 

 core of the block, but even this was very much modified. Such a core 

 looks like a pebble in a matrix, and gives the rock a conglomeratic appear- 

 ance. Still farther away tlie alteration was less, following only along 

 certain of the most jai'ominent parting planes, and leaving the sediments in 

 bands. 



PETEOGRAPHIC CHARACTERS OF THE METAMORPHOSED SLATES. 



Microscopic characters. — It would require a very much more detailed 

 study of these metamorphosed slates than has yet been made to enable one 

 to describe fully the various changes through which the original sediments 

 have passed in becoming the various kinds of rocks above described, and 

 numerous chemical analyses would be required in order to determine just 

 what, if any, changes in chemical composition had been produced by the 

 contact action of the gabbro. 



The chief constituents of these contact rocks are: Plagioclase feldspar, 

 little quartz, biotite, muscovite, chlorite, green, bluish, and brownish 

 hornblende, light-green pyroxene, hypersthene, olivine (?), titanite, epidote, 

 garnet, and magnetite. The mica-, hornblende-, and pyroxene-schists and 

 gneisses derived from the slates by the contact action of the gabbro almost 

 invariably contain very little quartz, but are full of a rich-brown mica 



