ELY GREENSTONE. 141 



The combination of these two characters and their general distribution 

 among the greenstones seem to indicate that these greenstones, in great 

 measure at least, were poured forth at the surface. 



THE SPHERTJLITIC STRUCTURE. 



The Ely greenstones are very frequently marked by small, rounded, 

 raised areas, which differ in color from the matrix, being' either lighter 

 or darker. They range in size from that of a pin's head up to 3 inches 

 in diameter. No structure is visible on the very small areas. They 

 merely stand out on the weathered surface of the rock as so many small 

 nodules, their relief being the result of differential weathering. On the 

 larg'er bodies, however, a distinctly radial arrangement can be seen, and 

 this is especially well shown on weathered surfaces. The essential 

 characteristic of spherulites is that they are formed of radiating or 

 diverging groups of crystals which commenced to crystallize from one 

 point or a center. These are the characteristics of the objects mentioned. 

 They are similar in general characters to the spherulites of the acid lavas, 

 but differ from them in mineralogic, and hence, of course, in chemical, 

 composition. One can see with the naked eye that chlorite in radiating 

 fibers is the chief constituent of some of the spherulites. Most of them 

 are formed of a grayish material whose character can not be recognized 

 macroscopically. The characters of these spherulites as seen under the 

 microscope will be described below (see p. 152). From the published 

 descriptions of various occurrences of spherulites it appears that fliey 

 are generally found in rock masses that are believed to be flows, or, more 

 rarely, upon the selvage of small dikes. So true is this that spherulites, 

 like porous and slaggy structures, have come to be considered as fair 

 evidence of the original extrusi^^e character of rocks in which they 

 occur. This spherulitic structure is not found, however, accompanying 

 the amygdaloidal rocks in the Vermilion district. On the contrary, the 

 conditions for the formation of gas pores in large quantity seem to have 

 precluded the formation of spherulites, although some few vesicles may 

 occm- with the spherulites. On one good exposure a traverse showed 

 the fine-grained amygdaloidal rock grading downward into a rock growing 

 gradually coarser and coarser, in which the amygdules disappear, and when 

 they had completely disappeared the spherulites were found to have 



