108 THE CRYSTAL FALLS IRON BEARING DISTRICT. 



the augite-andesites, and placed on the border hne between them and the 

 plagioclase basalts. It is preferred to hiclnde them under the basalts, though 

 it can not be doubted but that if analyses of perfectly fresh rocks could be 

 obtained, there would be found some which would incline more decidedly 

 toward andesites than do the above specimens. 



VAKIOLITIC METABASALTS. 



Variolites are spherulitic basalts, usually very vitreous. Since the tend- 

 ency to crystallization is so much stronger in the basic than in the acid 

 rocks, it is not surprising that they should be far less common than the cor- 

 responding acid kind. Moreover, the basic glasses are very susceptible to 

 alteration, which u,aturally obscures the original characters of the rocks. 

 This probably partly accounts for the fact that they are very infrequently 

 observed. This sphervilitic phase of the basalts is well known in Europe, 

 but there has thus far been .found only one reference to its occurrence in 

 the United States. Ransome^ has described a variolite from Point Bonita, 

 California. To this there may now be added a single occurrence in the 

 Crystal Falls district of Michigan. This variolite exposure occurs at N.375, 

 W. 900, sec. 4, T. 44 N., R. 33 W., in close proximity to the remnant of a 

 basalt stream which shows well-marked flowage structure. The relations 

 of the two rocks are not determinable from the exposures. 



The rock presents a very rough mammillated surface, due to differential 

 weathering. The varioles, being more resistant than the groundmass sur- 

 rounding them, form the protuberances. These protuberances vary in shape 

 from round to oval, and very rarely are irregular. The varioles vary also 

 in size from nainute ones to those about one-half inch in diameter, and con- 

 stitute by far the greater part of the rock. These general characters may 

 be seen on the photograph, fig. A, PI. X, taken from the hand specimen. The 

 color of the weathered surface of the rock is gray or light brown, while the 

 fresh surface is in general a dark green. Upon the polished surface of a 

 fresh rock the varioles have an olive-green color, with, in the majority of 

 cases, a distinctly darker center of purplish color. Less frequently this 

 center is lighter green than the remainder of the variole. The varioles are 

 usually separated from each other by narrow areas of groundmass, darker 

 than the varioles themselves, with a purplish or very dark olive-green 



iThe eruptive rocks of Point Bonita, California, by V. Leslie Ransome: Bull. Dept. Geol. Univ. 

 of Cal., Vol. 1, 1893, p. 99. 



