400 C. A. Vanhise— Enlargements of — 
from those portions of the petintecice alot in contact with 
overlying basic eruptives. This location is evidently a favor- 
able one for the development of such enlargements, the heate 
alkaline waters which would naturally descend from the over- 
lying lavas supplying approprts conditions. Then too, quartz 
enlargements when most easily found, are shown by lines o 
ferrite about the nuclei, id are ordinarily best seen in the less 
indurated quartzites. The Keweenawan cere are highly 
ferruginous, and are of an open texture ; hence, if among them 
feldspars have taken new growths, the pouatine for their de- 
tection are favorable. 
Fig. . Part of section of sandstone from sre Harbor, Mich., x 100; in polar- 
.& steed light. AAA, fra s, each from a single fe ldspar indivi idual ; : 
films of iron oxide on a nh ers of es agen grains; BBB, secondary : 
pag secre of the original B ewet ©, quartz grains; DD, unfilled s spaces 
EE, secondary fe soniye: grains pola ee independently ‘of the ware grains. 
with the vended a waht. effervescence. in thin section the. 
sandstone is seen to be composed largely of grains of different 
feldspars, next to which in abundance are rounded complex 
fragments derived from a granitic porphyry,* consisting of 
*The Copper Bearing Rocks of Lake epee ty £ h. oe Third An- 
nual a — States Geological Survey, p. 1 eae 
s 
£ 
