■■?t 



118 



BULLETIN OF THE 



acidic volcanic material, from which nearly all the other parts of the 

 rock have been removed by percolating waters ; especially as other sand- 

 stones have been said to be composed of quartz crystals. 



As the sedimentary rocks are more and more studied, the evidence 

 comes on every side that, like this sandstone, while their formation may 

 have taken as long as is generally supposed, they may have been de- 

 posited very rapidly, some being composed of old volcanic scoria, ashes, 

 and mud. In very many other cases the supposed sedimentary rocks 

 are really volcanic flows or intrusive masses. In the sandstone just de- 

 scribed the same masses of clay (460) occur as on the Douglass Hough- 

 ton Eiver, and they may arise here, as there, from the decomposition of 

 the enclosed feldspathic or argillite pebbles. Another solution of the 

 question of the origin of some of these would be the filling in of cavities 

 formed by the removal of some other material, by the argillaceous mate- 

 rial brought from above. This is suggested 'by the finding of stalactites 

 of the sandstone (464) extending down into the clay. At the spring just 

 above the quarry the sandstone (521) is red spotted with white, and 

 dipsN. 45°W. 14^ 



The relations of the traps to the interbedded sandstones and conglom- 

 erates have been given before. The trap has, when covered by the sand- 

 stone, been worn by water, and the latter rock deposited upon it. The 

 sandstone^ in its lower portion, holds to a greater or less extent the 

 debris of the underlying trap. Should we apply then rigorously the 

 logic of the Michigan and Wisconsin geologists, we should make a dis- 

 tinct geological age every time a bed of sandstone or conglomerate was 

 formed over any of the trappean flows, 



would prove that the Keweenaw series is made iip of some forty to sixty 

 different geological ages, every one as distinct from the age of the rocks 

 underlying it, as they would niake the Potsdam sandstone distinct from 

 the Kewcenawan. 



Besides the melaphyr pebbles and detritus, the conglomerates are 

 composed of similar pebbles to those found in the conglomerate of 

 the eastern side. No. 552, from the Hecla mine, has a dark reddish- 

 brown groundmass, holding white and pinkish feldspars and quartz. In 

 the thin section it is seen to be an old rhyolite, and to have flowed as a 

 lava, for it possesses the contorted, twisted, fluidal structure seen in so 

 many of the rhyolites from the Cordilleras, with which it can be per- 

 fectly parallelized, except so far as the alteration has removed some of its 



Their method of roasonine: 



original characters. 



It shows the same twisting and interweaving of the 



1 



.4 



r 



brownish^colored glassy material that they do ; its quartzes are fissured 



