SOURCE OF PALEOZOIC SEDIMENTS. 1 i :> > 



important question of the source of the sediments composing those strata. 

 If such a wide-spread formation as the rocks of Niagara age was deposited 

 over the surface of the Archean " nucleus," as well as over the regions which 

 encircle it, it is clear that the Archean " nucleus " could not have been the 

 source of supply of those sediments. Some other portion of the continent, 

 or some other region now submerged, must have constituted the dry land of 

 that time. Where that region lies is a question yet to be answered. 



DISCUSSION. 



Professor J. W. Spencer: The facts set forth in this very interesting 

 paper by Dr. Lawson have their counterparts in the geological structure of 

 the South. The hummocky and rounded rock surfaces have always had an 

 interest for me, on account of their common occurrence in regions which 

 have been glaciated, and hence regarded by many as evidence of glacial 

 erosion. But in the paper of Dr. Lawson we learn that such surfaces existed 

 before the formation of the early Paleozoic terranes. Some of you may be 

 familiar with Stone Mountain, about fifteen miles from Atlanta, Georgia. 

 This is a rounded granite hummock of over a mile, in longer diameter, rising 

 700 feet above the plain. The rock is remarkably free from joints, and is 

 rarely traversed by even an insignificant vein. Thus its structure has been 

 favorable to the preservation of the rounded form, whose outline is as perfect 

 as any of the domes of glaciated Norway or Canada ; or of southeastern 

 Missouri, which lies outside of former glacial action. Stone Mountain rises 

 from beneath very much disturbed strata of gneiss, whose beds dip to the 

 southeast, and there is no gradation of any importance between the granite 

 and the gneiss. The gneiss is decayed to a depth, in some places, of at least 

 sixty feet ; but the granite is compact, without being weakened by even 

 incipient decay. The surface materials, as fast as decomposed, are washed 

 off by the rains. Thus the contrast between the two formations of rocks is 

 preserved. This Stone Mountain is only one of many in Georgia and Ala- 

 bama. Here, then, we have, in the South, pre-Paleozoic surfaces as old as 

 or older than those described by Dr. Lawson in the Lake Superior region, 

 and brought to light by simple atmospheric action. Along the Potomac 

 river we find hummocks being formed by the progress of atmospheric in- 

 vasion along lines of joints, but these are now in process of formation, and 

 do not represent so ancient surfaces as those of the granite hummocks of 

 the South. 



