THE OLARKSBUIJG FOIIMATION. 479 



other fragments are also more or less rounded at times, hut more frequently 

 they are shari)ly angular. 



The horubleude is of the green variet\' common to the heds already 

 described. It occurs in the usual large grains, which sometimes are idio- 

 morphic in cross-section, sometimes irregular in shape, and always more 

 or less cellular. The grains occur independently, lying in all azinuiths 

 in the schistose matrix, or they are grouped together into little sheaf-like 

 bundles. They are nuich more abundant in some bands than in (ithers, 

 often occurring so thickly as to exclude from them all biotite. In other 

 bands no am])hibole occurs, and in these biotite is abundant. Moreover, 

 in these bands the quartz grains are nuich more fragmental-looking than 

 those in the hornblendic bands, and besides there exists between theur a 

 very fine grained aggregate of quartz and plagioclase, mainly the former. 



The fragments in the schist-conglomerates require no special mention. 

 They are pieces of the tuffs and sediments, interstratified with the conglom- 

 erates, or of the ores and quartzites of the Marquette series l:)elow the 

 Clarksburg formation, or of greenstones that may have been portions of 

 interleaved lava flows, or perhaps portions of dikes occurring in the pre- 

 Clarksburg beds, or, finally, fragments from the Basement Complex. Some 

 of the fragments are waterworn, while others are sharply angular. 



The explanation of the schistose conglomerates is that they were orig- 

 inally beds made up of alternating layers of sediments, tuffs, and mixtures 

 of these, in which were embedded bowlders and pebbles of preexisting 

 rocks and irregular fragments ejected from the volcanic vent. Some of 

 these fragments must have been portions of the walls of the orilice through 

 which the eruption took place, for they are certainly pieces of the rocks 

 that constitute the Clarksburg formation. True volcanic bombs have not 

 yet been recognized, though it is possible, and, indeed, probable, that some 

 of the lustrous black fragments embedded in these conglomerates are of 

 this character. These beds were rapidly hardened and afterward made 

 schistose by mashing. Since the biotite flakes wind about the garnets, it 

 is concluded that these minerals formed before or during the mashing. 

 After this, contact action or later metasomatic change resulted in the 

 production of the amphibole. This is shown by the fact that the small 



