BASIC YOLOANICS OF HEMLOCK FOEMATIOX. 125 



It is also of interest to notice that there is a total absence of indica- 

 tions of copper in these Huronian volcanics, as well as in those of tlie Penokee- 

 Gogebic, although it is associated with similar rocks in the areas above 

 referred to as well as in many others. 



The amj^gdules, with the exception of those of chlorite and of biotite, 

 are of much lighter color than the body of the rock, and from a short dis- 

 tance give the rock the appearance of a. porphyry. Weathering gives the 

 rock a diiferent appearance according to the materials filling the vesicles. 

 Whei-e these weather readily they are removed and the rocks become 

 scoriaceous. Where, on the other hand, as frequently happens, the vesicles 

 are filled with quartz, the matrix weathers more rapidly and the rounded 

 quartz cores stand out on the face of the rock like the quartz pebbles from 

 the softer matrix of a conglomerate. 



In a few cases hematite is disseminated through the quartz of the 

 amygdules, giving it the l^right-red color of jasper, and by some these 

 amygdaloidal fillings have been taken for included jasper pebbles. 



Careful study was made of the filling of the vesicles with the object of 

 deterininiug the order of deposition of the minerals. However, it was found 

 that the amj-gdules in a single slide contain ver}- different fillings, one 

 chlorite, another calcite, a third epidote, and so on; and that even in the 

 same slide the relations are not always the same, a mineral which here 

 occupied the center of an amygdule being found there on the periphery. 

 Moreover, the same mineral species was found at times occupying the out- 

 side and the center of the same amygdule. 



It is clear that the fillings are not the result of a solution connnon to 

 all the lavas, but that the same kinds of solutions were active in the various 

 lavas at different times and even in the same lava, at different times. How- 

 ever, the conclusions reached were that the chlorite was generally the first 

 product deposited and the quartz usually the last. From the study of the 

 related amygdaloids upon Keweenawan Point, Pumpelly^ long ago reached 

 the conclusion that chlorite was the earliest product of alteration — hence we 

 may conclude the first to be deposited in the amygdaloidal cavities; and 

 that the latest mineral deposited in the cavities, omitting copper from 



' The parageuesis and derivation of copper and its associates on Lake Superior, liy Raphael 

 Pnmpelly: Am. Jour. Sci., .3d ser., Vol. II, 1871, p. 29. 



Metasomatic development of the copper-beariug rocks of Lake Superior, by Raphael Pumpelly : 

 Proc. Am. Acad. Arts and Sci., Vol. XIII, 1878, p. 307. 



