98 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



"apo," and it is unfortunate that these two terms should have been so nar- 

 rowly confined. As it is, the epi- and apo-basalts would be subordinate to 

 and therefore included under the metabasalts, as this term is used in this 

 report. In the first the production of secondary hornblende is character- 

 istic; in the second the process of devitrification, and hence the original 

 presence of a vitreous base, is the chief characteristic.^ 



METABASALTS. 



All of the basalts belong to the plagioclase type. They may be most 

 conveniently divided into nonporphyritic and porphyritic kinds, according 

 to their most obvious macroscopical characters. There has also been found 

 a single occurrence of a spherulitic basalt, which will be described under the 

 head " variolite." 



NONPORPHYRITIC METABASALT. 



The nonjjorphyritic rocks possess a fine-grained or aphanitic structure 

 and are amygdaloidal or nonamygdaloidal. There are included under 

 this general name the microophitic-textured fine-grained pre-Cambrian 

 basalts (diabases in part), the very amygdaloidal forms of the basalts 

 (spilites),^ and the melaphyres in part. In these rocks the former presence 

 of a considerable amount of original glass is probable, and they show the 

 various textures known as navitic, intersertal, pilotaxitic, and hyalopilitic. 



With the nonporphyritic basalts there have been included some rocks 

 which are to a considerable extent devitrified glasses, and others in which 

 only a few microlites have developed. These last two vitreous types occur 

 more especially in fragments in the tuffs, and are quantitatively unimportant. 



Petrographicai characters. — In color the uonporpliyritic basalts on fresh fracture 

 show various uniform shades of green, dark olive green usually prevailing. 

 Much less common are purplish-black rocks, and these are much more vari- 

 able in color. In one of them is seen lighter-colored schlieren, which pass 

 over into the ordinary dark colors. The lighter-colored portions are seen 

 on microscopical examination to be due to a smaller quantity of the iron in 

 them and to a greater quantity of chlorite than occurs in the rest of the rock 



' The above discussion was written and the determination to use the terms porphyry — without 

 textural significance, as in rhyolite-porphyry — metabasalt, etc., was reached, in 1896, before the com- 

 mittee on petrograpliic nomenclature of geologic folios was appointed by the Director of the United 

 States Geological Survey. 



'Microscopic characters of rocks and minerals of Michigan, by A. C. Lane: Rept. State Board of 

 Geol. Survey for 1891-92, 1893, p. 182. 



