A REEF IN TIIK POTSDAM OCEAN. 105 



of the Archaean rocks, whih> the mass of the formation is uniformly a coarse 

 sandstone, the product still of shore waves, but d('[)osited fartlun- from the 

 shore. While in the base of the formation fragments may b(! found of nearly 

 all of the harder rocks of the Archaean, its mass is composed of quartz 

 grains without any admixture of other rocks. 



We may thus, from a study of the Potsdam rocks and their relations, 

 infer with a high degree of probability that at this earl}- time the Black 

 Hills were already marked out, and that they stood above the waves of the 

 Potsdam shallow sea, probably as a long low reef or island. This reef was 

 undoubtedly as long as we now find the expo.sure of the Archaean rocks, 

 and may even have been of greater length, as we do not know the charac- 

 ter of the unexposed Potsdam of the Hills. 



Again, the Archaean rocks Avere in Potsdam time metamorphosed to 

 nearly or quite the same extent as now; for the fragments composing the 

 cono-lomerate are of th.e same character as the still unbroken strata of the 

 metamorphic slates and schists. The slates were also tilted to their present 

 high inclination, for upon their upturned surfaces the Potsdam rests imcon- 

 formably, and if any tilting of the metamorphic rocks had taken place 

 since the deposition of the Potsdam the evidence would be found in great 

 breakings and fractures in the sedimentary rocks. 



At the beginning of the Lower Silurian term we may hence imagine 

 the Black Hills, and possibly a much more extended region, as an island 

 ("an island" because the conglomerate is on both sides of the present 

 axis) — a reef of schists, quartzites, slates, and granites, running northwest 

 and southeast. Barren and desolate we ma}' picture this island, for we 

 know of no plants nor land animals that then had their existence. The 

 only moving things that left their record were the waves that rolled 

 over a broad and shallow sea and broke the silence by dashing against the 

 primordial land. Slowly but sm*ely they tore and undermined its cliffs and 

 rolled awav the frao^ments to form the congflomerates and sandstones of 

 another age. The inequalities of the Archaean shore became gradually 

 filled up, and as the sea rose higher upon the land all that was not worn 

 away at last became entirely covered bv the Potsdam sea and its sediments. 



That the Potsdam stretched completely across the present area of the 



