350 THE PENOKEE IRON BEARING SERIES. 



Microscopkal character of the diabases. — The diabases usually have a 

 well developed ophitic. structure, the augites being of large size and includ- 

 ing many somewhat idiomorphic lath-shaped plagioclases. In the diabases 

 in which tliis structure reaches the extreme the feldspars have a tendency 

 toward two generations, there being, aside from the smaller lath-shaped 

 plagioclases, larger, somewhat porphyritic appearing ones. The rocks vary 

 from ophitic diabases to a true gabbro, all grades of variation being 

 observed. In the ])assage from diabase to gabbro the feldspars become 

 broader; the pyroxene includes less of feldspar, until in the coai'sest grained 

 rocks the structure is granitic (granular) and the contained pyroxene takes 

 on the diallage cleavage. The gabbro occurs in only a few localities and 

 is of little importance as compared with the diabases. 



The original minerals are apatite, magnetite, olivine, plagioclase, and 

 monoclinic and orthorhombic pyroxene. The latter occurs only in one ex- 

 posure, and in the most widespread phase of rock the only important orig- 

 inal minerals are magnetite, plagioclase, and augite. The order given is 

 that of crystallization. In some of the rocks this succession can be made 

 out with a good deal of sharpness, each mineral present having nearly 

 completed its crystallization before the succeeding one began to separate. 

 This is particularly true of the ophitic diabases and becomes less and less 

 true in passing toward the gabbros. 



In the minerals present and their.relations there is nothing particularly 

 different from other occurrences of diabase in the Northwest with the 

 exception that an orthorhombic ])yroxene is found. The points of most 

 interest are found in connection with the alteration which the rocks 

 have undergone. iVs has before been said, they vary from an almost 

 perfectly fresh condition to one in which none of the original minerals 

 reinain. The rock in which the alteration has gone farthest is contained 

 in tlie lron-l)earing meiuber, and as this peculiarly altered rock is different 

 from the ordinary altered rocks of the series it will be separately considered. 



The ai)atite is but sparsely jiresent in most of the rocks, but is occa- 

 sionally found in (piite plentiful large, well formed crystals. It is always 

 the first mineral to crystallize and has the usual well developed crystal out- 

 lines. 



