870 THE GEOLOGY OF MINNESOTA. 



[Porphyry. Conglomerate. 



Mic. The feldspar, which appears to have been in part a soda-lime species, is 

 much altered by the development of mica, calcite and epidote, but has also been, in 

 some cases, recrystallized by new growths about the margins, and more or less 

 throughout its entire mass. Rarely a feldspar is embraced wholly within a horn- 

 blende, but as a rule these minerals are in independent idiomorphic relations. 



The hornblende is green, distinctly pleochroic, having the maximum extinction 

 angle, on elongation, about 16. It is sometimes zoned with different shades of 

 green. It has a negative acute bisectrix (n,,), and n f nearest the vertical axis, and is 

 hence apparently common hornblende. 



The matrix is composed largely of very fine feldspars, micas and epidote, with 

 some calcite and shreds of hornblende. One section. 



Age. Intrusive in the Upper Keewatin. 



Remark. This rock, in thin section, has the appearance of having originated 

 from a hardening of a debris of feldspathic character. The alteration of the feld- 

 spars took place while they constituted a part of such debris. N. H. w. 



No. 2245. CONGLOMERATE. (Granituxd.) 



Part of the same rock as No. 2244. 

 Ref. Annual Report, xxiv, page 76. 



Meg. Shows variations of grain due to a pebbly structure. The pebbles, while 

 mainly of a porphyry, are sometimes of a dark, hornblendic rock, and the face of the 

 rock, where weathered, occasionally shows also a sedimentary structure. The por- 

 phyry pebbles differ in the coarseness and in the frequency of their phenocrysts, in 

 a manner like the differences seen in the pebbles of the Stuntz conglomerate. In 

 this case, however, the crystals are of feldspar. 



Mic. This rock also presents the appearance of a hardened clastic debris iden- 

 tical in history and origin with No. 2244. The grains of hornblende and feldspar 

 are smaller, and more like fragments and shreds than phenocrysts. This debris 

 contained also a little quartz. They also grade more regularly into the size of the 

 grains of the matrix. One section. 



Remark. "On looking about over these knobs it appears that this rock is 

 generally finely porphyritic with feldspar, and had originally pebbles of porphyry 

 and fragments of a dark rock, constituting a conglomerate, showing in spots traces 

 of a sedimentary structure, and really is but a condition of some parts of the frag- 

 mental formation. Yet it appears like rock No. 2243, and is massive as granite, 

 having angular cross-jointage. The appearance and action of this intrusion are quite 

 similar to the same in some of the Kekequabic Lake granite. Nos. 2243, 2244 and 

 2245 constitute a series showing what outwardly indicates intrusive and igneous 

 action of a rock that originally was fragmental, and which still retains (in No. 2245) 



