3*84 W. J. McGee — Three Formations of 



region in the north ; this general mantle merges into the 

 elevated deltas of the fluvial phase on the one hand, and into 

 the modern alluvial, estuarine and marine deposits on the other ; 

 and it is either buried beneath or broken up and incorporated 

 within the terminal moraine in its northward extension. 



Sources of Materials. — The prevailing materials of the de- 

 posit may be roughly classed as (1) well rounded cobbles and 

 pebbles, such as those of northern New Jersey ; (2) well 

 rounded quartz and quartzite gravel, such as overspreads penin- 

 sular New Jersey ; (3) fine gravel and sand, clean or intermixed 

 with clay or silt, forming a matrix in which the coarser 

 materials are imbedded, and constituting the great bulk of the 

 deposit, particularly in the south ; (4) loam, resembling that of 

 the upper division of the fluvial phase, and exemplified by the 

 high level red loams of northern New Jersey and southeastern 

 Pennsylvania ; and (5) clay and silt, generally stratified and 

 limited to low altitudes. The sources of the first two of these 

 classes are precisely, and those of the next two proximately, 

 determinate. 



1. The cobbles of northern New Jersey are in form and 

 material identical with, and in size generally smaller in a 

 graduating series than, the high level bowlders of the fluvial 

 phase along the Delaware; both are identical litho logically 

 with the axial quartzites of the southeasternmost Appalachian 

 ranges ; and both cobbles and bowlders not infrequently con- 

 tain fossils identical with those of the quartzite ridges. The 

 erratics, it is true, are commonly more profoundly metamor- 

 phosed than the parent ledges ; but they obviously represent 

 the most obdurate portions of these ledges, and moreover 

 examination shows that in many cases the metamorphism is 

 superficial and produced by interstitial growth after the manner 

 described by Irving, while the interior remains in the same 

 condition as the quartzites now found in situ. Some of these 

 cobbles doubtless formed originally a part of the Potomac 

 formation, and were removed from it and redeposited during 

 the Columbia epoch ; but others appear to have been derived 

 directly from the quartzite ranges whose bases were washed 

 by the floe-bearing Columbia waters, and whose ledges were 

 shattered by the Columbia cold. Certainly the angular blocks 

 now cumbering the upper mountain slopes, and the smaller and 

 well-rounded cobbles imbedded in the Columbia formation, are 

 but the extremes of a graduated series whose continuity can be 

 readily traced along either the Delaware or the Lehigh. 



2. Save that its average size is slightly smaller, the well- 

 rounded gravel of peninsular New Jersey and the Maryland- 

 Delaware peninsula is identical in all physical characters with 

 that of the Potomac outliers skirting the Piedmont escarp- 



