220 THE GEOLOGY OP MINNESOTA. 



[Aporhyolyte. 



Urn/ark. Taken together, and considering the manner of association, the two 

 rocks here described probably represent the great diabase sheet already mentioned 

 (No. 143) near its contact with the "red rock " Cabotian, and included fragments of 

 the " red rock." It is noteworthy, also, that the red rock here included is an apob- 

 sidian, rather than an aporhyolyte, thus agreeing with the character of the red rock 

 known to exist near this place, viz., the pebbles in No. 141). N. H. w. 



No. 14"). APORHYOLYTE. 



Baptism river, third fall. This rock is somewhat finely amygdaloidal and weathers into rough slates, which 

 are again cut by joints into lenticular pieces that present their sharp corners as their neighbors fall out. These 

 slates have a dip north. Indeed, the rock No. 145, so far as it appears along the river here, has shown a dip 

 northwardly, but only occasional exposures occur, generally low and water-covered. The rock here rises above 

 the top of the falls about thirty feet, the whole hight being 10o feet. 



Ref. Annual Report, ix, page 37. 



An aphanitic, hard, pinkish-gray rock with numerous phenocrysts of 

 quartz, and white, more or less decayed, feldspars. On a perfectly fresh fracture 

 incipient cracks are noticeable and these are of a white color. 



Mic, The groundmass of the rock is essentially like others of the aporhyolytes 

 already described (see Nos. 138 and 68), and the areas of poikilitic qwuiz are quite 

 large. The quartz phenocrysts are similar to those in the other specimens described 

 (Nos. 138, 139, 140) and some of these crystals are distinctly bipyramidal. Frequently 

 the quartz of the groundmass adjoining a quartz phenocryst has the same orientation 

 as the phenocryst. The porphyritic feldspars are quite frequently partly replaced by 

 mli-ite. Usually these feldspars break up into numerous small irregular areas of 

 darker and lighter shades. This appearance is especially prominent when near the 

 position of extinction. There seems to be in each crystal only two orientations for 

 these patches, and they extinguish very nearly together. Occasionally a fine twin- 

 ning, probably albite, is seen in parts of the feldspars, and in such cases the extinc- 

 tion is closely parallel to the twinning lamellae. A grain which gave a positive 

 bisectrix almost perpendicular had an extinction of about 5, but such a result is not 

 distinctive as an almost similar result could be gotten from orthoclase and oligoclase. 

 The closely parallel extinction, the twinned grains and the presence of calcite as an 

 alteration product would seem to point to oligoclase. It thus seems that we here 

 have an intergrowth of orthoclase and oliyocltixi: 



There are a few small areas, now filled with nii/i/<'t!fr, licnxiiHc and ritlor/tc, 

 which seem to represent old phenocrysts of a ferro-magnesian mineral, but what it 

 was is uncertain. One grain of nit He was seen. 



Three sections. 



Age. Cabotian. 



Remark. The aporhyolyte at the third fall of Baptism river (No. 145) is evidently 

 a part of the same mass that forms the Great Palisades (Nos. 138, 139, 140). u. s. G. 



