1888.] 497 [Crosby. 



quartzites agrees very closely with the Taconian system of west- 

 ern New England. In fact, so far as the rocks named are con- 

 cerned, the correspondence may be described as exact. The 

 conglomerate is, perhaps, more prominent than in the typical Ta- 

 conian ; but scarcely more so than in the series of semi-crystalline 

 rocks of Trinidad and the Spanish Main, which the writer has else- 

 where correlated with the Taconian. The specular schists and 

 slates of the Black Hills are matched exactly by the itabarite, 

 which is such an interesting lithological feature of the Taconian 

 in South Carolina and in South America. The most important 

 discrepancy noticed between the newer metamorphic series of the 

 Black Hills and the Taconian system is the absence of crystalline 

 limestones or marbles in the former. We have some evidence, 

 however, that this discrepancy is not real. It is necessary to 

 remember that only a part, perhaps a small part, of these meta- 

 morphic terranes is exposed in the Black Hills. To the north 

 and east the ancient eroded surface of the newer series is covered 

 by the Potsdam and higher formations, perhaps for hundreds of 

 miles. That the still concealed portions of this series do include 

 some limestones seems to be placed beyond reasonable doubt by 

 the following observations : In the quite widely separated canons 

 of Spring Creek and Bear Butte Creek, which flow eastward from 

 the newer Archaean series through the overlying formations, I 

 found that portions of the conglomerate forming the base of the 

 Potsdam contain numerous waterworn pebbles of a finely crystal- 

 line white or grayish- white limestone. No strata from which these 

 pebbles could have been derived are exposed anywhere in the 

 Black Hills ; and the natural inference seems to be that the peb- 

 bles represent beds of limestone intercalated with the slates and 

 quartzites, which are the source of the remaining pebbles of the 

 Potsdam conglomerate, but which are still covered by the Pots- 

 dam and other Palaeozoic strata. 



I am aware that investigations now in progress threaten, as it 

 were, to discontinue the original Taconian system as an Archaean 

 terrane. But this result, even if finally established, cannot inval- 

 idate the lithologic correlation proposed here ; although it would 

 necessarily deprive it of much of its geological significance. 



The western or older Archaean series of the Black Hills cannot 

 be distinguished lithologically from a considerable part of the 

 great series of mica schists and micaceous gneisses forming so 



PROCEEDINGS B. S. N. H. VOL. XXIII. 32 JUNE, 1SSS. 



