232 Van Hise — Enlargements of Hornblende Fragments. 



Hornblende is one of the most important of the remaining 

 rock-forming minerals. The object of the present paper is to 

 describe briefly some unmistakeable enlargements of horn- 

 blende fragments that I have recently met with in studying 

 certain peculiar conglomerates under the direction of Professor 

 E. D. Irving. 



At Ogishke Muncie and Cacaquabic lakes, in the northeast- 

 ern part of Minnesota (T. 66 N., E. 6 W., and T. 66 K, E. 7 

 W.), occurs a great belt of conglomerate forming part of the 

 Archaean schists of that region. The following extracts from 

 field notes by Mr. W. M. Chauvenet, of the IT. S. Geological 

 Survey, are descriptive of this conglomerate : On Ogishke 

 Muncie "the great conglomerate formation or bed shows no 

 apparent dip or strike, but massive walls of green color with 

 joints and great cracks. . . . The pebbles protrude in masses, 

 the great pink, granite pebbles being most numerous, often as 

 large as three feet through. Bright red jasper pebbles give the 

 rock a handsome appearance. Quartzite and slate pebbles are 

 also present." At another place on Ogishke Muncie Lake: 

 " The conglomerate and greenish conglomeratic rock rise in 

 contact forming a cliff 30 to 40 feet high, broken or irregular. 

 There is no marked division between the two, the one blending 

 with the other. ISTo red jasper pebbles are here seen, which 

 fact distinguishes this from the coarser conglomerate. The 

 pebbles are black and green in a dense matrix of crystalline 

 greenish rock." At Cacaquabic Lake: "The conglomerate 

 appears on the shore. The conglomerate differs little in 

 appearance from that at Ogishke Muncie. The pebbles are of 

 many kinds but not so thickly scattered. Jasper pebbles are 

 very common, granite, clear quartz, and slate pebbles are pres- 

 ent. This was especially apparent upon weathered surfaces." 

 At times the exposures become " a green, sub-crystalline, 

 exceedingly tough rock, but appearing like a bedded sandstone 

 in place. It is finely but distinctly banded, cleaving most 

 readily along the bands. . . . Above this rock is a distinctly 

 crystalline, greenish rock, weathering to green and red, having 

 a conglomeratic appearance in places where red granitic grains 

 are thickly scattered." 



The best illustrations of the enlargements of hornblende yet 

 found are in the denser parts of the conglomerate where the 

 pebbles are comparatively rare. But these enlargements are 

 present in nearly all sections cut from the matrix of this con- 

 glomerate throughout its entire known extent of a number of 

 miles. As is to be expected, thin sections from different local- 

 ities are quite different in appearance and composition. In 

 many cases the finer parts of the sections are so exceedingly 

 fine as to make it difficult to determine accurately the various 



