11;8 TUE CRYSTAL FALLS IKOX BEARING DISTRICT. 



conunon. These crystals give a, secondary porphyritie character to the hivas, 

 and the microscopical appearance of the rocks varies somewhat according 

 to the occurrence of the calcite. Such rocks, for instance where the 

 rhombohedra occur, loolc on fresh surface by rapid examination like 

 porphyrites in which the feldspar sections are all quadratic. In the others 

 the scalenohedral sections resemble in general lath-shaped feldspar pheno- 

 crvsts h'ing scattered in all directions on the surface of the rock. 



Another case of extreme alteration is shown in a light greenish-gray, 

 much-altered schistose rock from sec. 21, T. 46 N., R. 32 W. Upon the 

 weathered surface long grooves are noticed — one measuring 60 mm. long 

 by 5 mm. wide — which on the fresh surface are filled with calcite. On 

 faces perpendicular to the long extension of such grooves they appear as 

 narrow slits, with the long direction of the slit, that is, the width of the 

 groove, agreeing with the schistosity. These are clearly flattened amygda- 

 loidal pores, and but for them the igneous nature of the original rock could 

 not have been determined. The extreme flattening of these amygdaloidal 

 cavities and the schistose nature of this rock produced from an original 

 volcanic, points toward mashing as one of the causes, if not the main cause, 

 of its present characters. It is now composed of fairly large autoraorphic 

 actinolite individuals, a very small amount of biotite and chlorite flakes, and 

 masses of grains of quartz, calcite, epidote-zoisite, magnetite, with ilmenite 

 and hematite iii thick plates filling in the spaces between the actinolites. 

 If any feldspar was originally present, it is now entirely concealed by the 

 calcite and epidote-zoisite. 



The calcite phenocrysts are found in the fairly fresh lavas. Thev are 

 beautifully automorphic and are certainly not replacement pseudomorphs of 

 some original phenocrysts, but replace the various minerals of the fine- 

 grained mass. Moreover, it is clear that they were formed subsequent to 

 all dynamic action, as their crystal outlines are perfect and the}' never 

 show any evidence of pressure. This is so even in those cases where the 

 amygdules which have been markedly elongated are filled with calcite 

 The pi'ocess of replacement could not be followed, but it is evidently con- 

 nected with the development of chlorite, those rocks in which a great deal 

 of the calcite occurs having chlorite developed instead of actinolite. 



In other sections in which the amount of porphyritie calcite or calcite 

 and mnscovite is much greater than in the rocks just described, the amount 



