200 PALAEOZOIC TIME — LOWER SILURIAN. 



as the Huronian period in the Azoic. The depression may not 

 have begun at that time ; but the subsidence attending the forma- 

 tion of the thick deposits was an early one in the series that ended 

 in giving the main features to the present lake-region. 



(7.) Atlantic currents. — From the map, p. 136, it is apparent that 

 the northern oceanic current of the Atlantic would have traversed 

 New England, New York, and the Appalachian region to the south- 

 west, while the warmer Gulf Stream would have in part followed its 

 present course and partly have flowed over the Grulf of Mexico and 

 up the interior sea of the continent. 



2. Origin of the material of the rocks. — (1.) /Sandstones, shales, 

 and conglomerates. — The Azoic age had left the earth with a surface 

 of rock more or less covered with gravel and sand both above and 

 below the water. The waves, running streams, and slow wear and 

 decomposition through atmospheric causes, were working out their 

 legitimate results during the closing part of the age, as well as 

 earlier when the Azoic rocks themselves were accumulating. The 

 surface could not have failed to be extensively spread with earth, 

 and, at the opening of the new age, this earth or gravel would have 

 made the sand-flats and higher fields that were exposed over the 

 half-submerged continent, as well as the bottom of the seas. 



From this material and the accessions derived through subse- 

 quent wear and decomposition, all the Potsdam beds and all later 

 rocks of mechanical origin (exclusive of limestones) have to a great 

 extent been made. This material has been worked over again and 

 again, the accumulations of one age being in part distributed anew 

 to make the rocks of a later; and by this means the geological 

 series, with the exception stated, has been in the main built up. 

 The rocks of igneous origin have added to the stock only an in- 

 considerable proportion of the whole amount, and those of che- 

 mical origin a much smaller fraction. 



The beds of sandstones, shales, and conglomerate of the Potsdam 

 period, we thus conclude, derived their material from the sand- 

 flats, from the higher gravel-fields which the encroaching waves 

 and running waters would level and carry off, and from the rocky 

 Azoic hills and mountains that were subject to degradation by 

 streams and decomposition. In an age without land-vegetation to 

 bind the soil, the degrading-process would have been more com- 

 plete than at the present time. These materials were spread out 

 in layers by the waves and currents over the continental regions ; 

 and the animals, which had a living-place in the mud or sand, 

 found there a burial-place also, to remain in many instances as 

 fossils, in attestation of the life of the period. 



