282 THE GEOLOGY OP MINNESOTA. 



[Basalt. 



ness. A single subrounded grain of zircon is greenish between crossed nicols, clear 

 and bright, and extinguishes parallel to its greater diameter. Its green color (the 

 section being about .03 millimeters in thickness) is in the third order. 



The fourth section, made from a pebble from this conglomerate, is finer grained, 

 but essentially a clastic quartzyte. The grains are interlocked by secondary quartz 

 growths, and these growths embrace many clouded impurities which served to make 

 up the original surrounding mass. Hence this interstitial silica is rather dark, espe- 

 cially between crossed nicols. Whether this secondary growth took place prior or 

 subsequent to the incorporation of these grains in this quartzyte is an important 

 question, for it cannot be distinguished from a poikilitic extension of secondary 

 quartz through an aporhyolyte. If it preceded the formation of the conglomerate, 

 there must have been surface rhyolitic rocks to serve as its source. It would not be 

 safe to affirm, on the evidence of this section alone, the existence of such a rock in 

 the Grand Portage region at the date of the formation of this pebble. 



Remark. The importance of determining the stratigraphic position of this 

 conglomerate precludes a discussion of the question at this place. It has been 

 supposed to lie at the base of the Keweenawan. It is similar to that represented by 

 No. 1903, which was found about the centre of N. W. J sec. 25, T. 64-4 E., south- 

 ward from South Fowl lake, and to that seen near Fond du Lac, in the valley of the 

 St. Louis river. It outcrops on the mainland in the hills about a mile west of Grand 

 Portage village, and is illustrated by figure 1 of this volume. N. H. w. 



No. 255. BASALT. ( Amyydaloidal. ) 



" This overflow comes down to the water at once and hides the conglomerate, and rises perpendicular about 

 twelve feet. It weathers very rough and open angular, from containing fragments, apparently, of rock from 

 contiguous formations, that were not wholly molten." Grand Portage island. 



Ref. Annual Report, ix, page 61. 



Meg. A very fine-grained, much decayed, greenish-gray rock, which holds small 

 amygdules of calcite. One side of the specimen has some gray shaly material and 

 also some of the underlying sandstone, indicating that this side of the specimen 

 was in contact with the sandstone or that it included a fragment of the sandstone. 



Mi<-. The groundmass of the rock is green, in fact the whole section is, due to 

 the presence of a large amount of chlorite. This groundmass is made up of calcifr, 

 clt in rite, small, black spots, and the remains of plugioclase microliths. There is also 

 a little hciinififr and jii/ritc. The small black spots are opaque, black in transmitted 

 light and gray in reflected light. There are the outlines of many porphyritic crystals, 

 the crystals having been replaced entirely by calcite, or chlorite, or both. In no 

 case was any of the original material of the crystal discernible. Some of these phen- 

 ocrysts show eight-sided sections which might be referred to cross-sections of pyrox- 

 enes, and others, by the arrangement of the chlorite, suggest olivines. There are 



