COKIIELATION. 63 



microcrystallinc coiulitioii. Then ciiiiic ii ^rcnt (lyiuiniic revolution, fol- 

 lowed by a gTcat erosion, and the result at the site of the Black Hills was 

 the exposure at the surface of a Ijroad section of vertical schists and slates. 

 Of the base of the schists and of the sunnnit of the slates we have no 

 evidence. 



Can we correlate them with the Arclucan rocks of the East? 



Primarily the historical order of rocks is shown by the order in which 

 they overlie each other, and since every change in the character of succes- 

 sive strata represents a historical change, the geologist always classifies his 

 section by means of these changes. But when remote sections, the con- 

 nection between Avliich cannot be traced by the eye, are to be compared, 

 and their historical relations are to be made out, or, in other words, when 

 correlation is attempted, the all but universal verdict of geologists is that 

 evidence from fossils is alone of value and that lithological evidence is not 

 to be trusted. Nevertheless there are some lithological correspondences so 

 general that they cannot be accidental, and they must be counted to give 

 a certain weight to lithological evidence in the absence of paleontological. 

 Triassic rocks are characterized by a red color the world over. Permian 

 limestones are always mag-nesian. The great limestones of all great rock 

 systems are Paleozoic In innumerable localities in all lands the Paleozoic 

 system begins with a conglomerate and the rocks beneath it are metamor- 

 phic. These agreements cannot be due to mere coincidences, and if they 

 are not, then lithological resemblances are not always without meaning, 

 and they may be taken to afford the basis for a presumption so long as the 

 better evidence from organic forms is not available. I shall, therefore, give 

 consideration to the lithological affinities of the Black Hills Archaean as 

 the best available means of judging of their relations. 



The rocks of the eastern division of the Archaean — the argillaceous 

 and silicious slates — have a very similar character to the Huronian rocks 

 of the south shore of Lake Superior and Canada. They agree in their 

 partly argillaceous character, in the presence of silicious slates, in the great 

 abundance of quartzite, and in the occurrence of silicious hematite, banded 

 quartz, and jaspery formations. We find, however, in the Black Hills no 

 diorites, conglomerate quartzites, nor limestones, and the iron deposits. 



