BASAL BEDS. 137 



later, as a result of renewed uplift of the land or of the deeper intrenchment 

 of streams and the incision of waves, that the fresh, undecomposed granitite 

 came into the grasp of the eroding agents, and, breaking up along planes of 

 fracture, made pebbles. In the meantime, there were portions of the granitic 

 terrane which did not disintegrate. These parts were the quartz veins and 

 nodular segregations of quartz in pegmatites, and to some extent, perhaps, in 

 dike rocks, which remained and formed pebbles. It is owing to this reason 

 that the first conglomerates overlying the arkose beds are composed mainly 

 of quartz pebbles. The considerable mass of these pebbles gives some idea 

 of the thickness of the decomposed granite section which was removed at 

 this time. In any section of the neighboring granitites at the present time 

 the volume of quartz large enough to form pebbles in a cubic yard of the 

 rock is relatively very small. Large and thick veins occur here and there, 

 but it is not conceivable that more than 1 per cent of the average granitite 

 mass would yield quartz pebbles. It is evident, therefore, that a very 

 thick section of rock was subjected to disintegration and removal in order 

 to form a bed of quartz conglomerate 50 to 100 feet thick. How much of the 

 quartz came from veins in sedimentary formations into which the granitite 

 doubtless penetrated can not be known. The quartz would probably be 

 more abundant in these rocks than in the granitic stock itself. That the 

 depth of granite removed to form these arkoses and quartz pebbles was 

 great is also indicated by the coarsely crystalline texture of the granitite 

 along the border, showing that the parts which we now see are well into 

 the interior of the original mass and not near the contact with the rocks into 

 which the granitite was intruded, for the reason that at the contact the 

 granitite would have cooled down more quickly and thus have induced 

 upon its crystals a more minute texture than exists in the mass, where 

 cooling went on more slowly. 



Geographical conditions indicated by the basal arkose. The formation of beds of ai'lvOSe, 



and the abundant reasons above cited for believing that the land on which 

 the Carboniferous beds were laid down had long been subjected to secular 

 decay and leaching, make it necessary to suppose that the grades of the 

 streams were too low for the removal of the products of disintegration as 

 fast as they were formed. The site of the basin, which appears to have 

 undergone depression at the time deposition set in, must have been pre- 

 viously without very strong contrast with the surrounding country, except 



