240 THE CRYSTAL FALLS IRON-BEAKING DISTRICT. 



DESCRIPTION OF INTERESTING KINDS OF GABBRO. 



The minerals described above as tlie leading essential constituents of 

 the rocks to be described may be combined in varying quantities. Accord- 

 ing to these combinations a number of different mineralogical types of rock 

 may be produced. The wide range in mineral composition of the gabbroic 

 rocks is equaled, if not surpassed, by similar variations noted by Fairbanks 

 in certain rocks from Point Morrito, California.' It may cause the further 

 desci'iption to be more readily understood if we preface it by the statement 

 that all of these types, however, are simple facies of a single magma. The 

 important phases which will be described are, in the order of their impor- 

 tance, hornblende-gabbro, consisting essentially of hornblende and lab- 

 radorite; gabbro, consisting of monoclinic pyroxene and labradorite, and 

 bronzite-norite, consisting essentially of bronzite and labradorite. The 

 vai'ious mineralogical types exhibit very interesting ranges in texture in 

 certain cases, to which attention will be called. 



HORNBLENDE-GABBRO IN SEC, 15, T. 42 N., R. 31 W. 



A hornblende-gabbro forms a large knob in sec. 15, T. 42 N., R. 31 W., 

 just at the foot of the Norway Rapids, on the west bank of the Michigamme 

 River. This exposure shows very prettily a change in texture. The change 

 in texture is also accompanied by a slight mineralogical change. The knob 

 is composed partly of a fine-grained granular, but more largely of a coarse- 

 grained porphyritic, gabbro. The fine-grained portion is a pure gabbro 

 composed of plagioclase and brown hornl:)lende, with very little brown 

 mica. No quartz was observed, nor was any orthoclase definitely deter- 

 mined. The plagioclase is in fairly well-de ^eloped automorphic plates. 

 The hornblende is the brown variety, with numerous minute inclusions, 

 which has already been described, and is not always so well developed as 

 is the plagioclase. In places it plays rather more the role of a cement. 

 This relation of the two minerals results in forming an imperfect ophitic 

 texture in places, though on the whole the two minerals are about equally 

 developed, and produce a granular structure. (Fig. A, PL XLIV.) 



' The geology of Point Sal, by H. W. Fairbanks : Bull. Dept. Geol., Univ. California, Vol. 11, 

 1896, p. 56 et seq. 



