586 DESOKIPTIVE GEOLOGY. 



choidal fracture. At a little distance, it might be mistaken for a basaltic 

 rock. An analysis of this ore, made by Mr. B. E. Brewster, gave the 

 following results: 



Ferric oxide 84.217 



Alumina 0.178 



Manganous oxide 1.454 



Magnesia 0.472 



Water 1.713 



Insoluble residue 12.518 



Total 100.552 



which would give a percentage of metallic iron 58.95. The occurrence of 

 such a body of iron-ore in the midst of volcanic rocks is an interesting phe- 

 nomenon, though it is probably too far from any source of fuel supply to 

 be of practical value. It is, however, a remarkably pure ore, being entirely 

 free from phosphorus or sulphur, though containing a rather high percent- 

 age of silica. 



It may possibly have some connection with a peculiar rock which is 

 exposed under the trachyte body on the opposite side of the river, forming 

 the cliffs of the west side of a ravine which enters the canon from the north. 

 This has a very distinctly columnar structure, and is evidently of earlier age 

 than any of the rocks around; but its relation to the other andesite bodies, 

 on account of its isolated position, could not be determined. It is a heavy, 

 fine-grained, crystalline rock, of a dark-gray color, whose constituents, with 

 the exception of the plagioclase-feldspar, cannot be distinguished by the 

 naked eye. Microscopic examination shows it to be a quartziferous augite- 

 andesite, being made up of plagioclase, and a little sanidin-feldspar, augite, 

 biotite, and some angular grains of quartz with glass-inclusions. Quartz 

 crystals and some apatites are also found imbedded in the larger feld- 

 spars. It contains no hornblende or olivine, nor has it any glassy or 

 amorphous base. Its chemical composition, as determined by Mr. Rein- 

 hard, at Leipsic, is: 



