292 G. r. WRIGHT rOSTGLACIAL EROSION AND OXIDATION 



receut ; so recent that at the places indicated there can be no distinction made 

 between the transported materials of the border and of the moraine. 



''The character of the orif/inal material of the glacial accumulations. — ^We 

 must agree that the ice could not attaclv the solid ontcrops until the soil rotted 

 in place had been removed. The depth of that soil varies according to ma- 

 terial, slope, and climate. Agassiz reports soil over 100 feet in depth along 

 the Amazon. 



"Before the solid, rockj' portion of the outcrops were reached there was 

 pushed before the glacier this oxidized soil without distinguishing character- 

 istic. I have advocated several times the study of the soil of the 'border' 

 under the microscope, as our petrographic knowledge is so sure that it may be 

 possible to detect and distinguish varieties in an apparent homogeneous body 

 of oxidized soil. 



"Next. — The glacial scrapings then were mingled witli the rotten portions of 

 the solid rock next the outcrops, and these were glaciated, as shown in the 

 pebbles found at Warren among the gravels, where the river action was strong, 

 and also in a cobble of gneiss found west of Irvineton on the top of the hill 

 and several hundred feet above the river. Here the solid nucleus was within 

 half an inch of one side and 3 inches from the opposite surface, showing 

 weathering before rounding, and the rounding was from ice rather than water, 

 as it was above stream action. 



"The slate region of the Great Valley of Pennsylvania, from the Lehigh to 

 the Schuylkill and utterly south of the Moraine, which was 30 miles to the 

 north, was a good field for inspiring patience. In the first place the slate was 

 so solid under the thin soil that post holes had to be cut into it for the fences, 

 and the posts had to be wedged in with cobbles collected here and there from 

 a region generally free from such things. 



"In this region there was a fresh slate surface, coming sharp and solid 

 under an oxidized deiwsit of soil, which was not slate soil. On this area there 

 were at times masses of the rocks of the crest of the Blue Ridge moved a mile 

 or more to the south and sometimes as large as a small house. At other times 

 I have followed a section which went to the solid slate, and in the uniformly 

 well rotted and fine soil there was not even a slate flake. Here was evidently 

 the original preglacial soil. After a mile or so of search along a section I 

 have found an Oriskany or Oneida pebble, and it was as fresh as pebbles 

 usually are that are in oxidized soil, and that means a thin yellowing stain on 

 the outside, but not extending inward more than a hair's breadth. Then I 

 would come upon a sporadic deposit of gravelly soil. 



"The influence of a rotted soil containing a large amount of iron peroxide is 

 shown in the pebbles (Olenellus quartzite) of the basal conglomerate of the 

 Triassic, just south of the South Mountain, at Hosensack, Pennsylvania. This 

 conglomerate has thoroughly rotted, and its pebbles lie in the red and loose 

 soil and very much resemble a glacial deposit: but the pebbles are as hard as 

 if rolled today and with the steely-blue patina of magnetic oxide. Their sec- 

 tion shows that the iron jienetrated more or less deeply, and in every case 

 there is no difficulty in picking them from a mixture of Olenellus pebbles from 

 more northern localities. 



"Olfl (jlacial deposits. — These must show a difference from fresh ones. The 

 characteristics of a fresh deposit are that its oxidized character shall not vary 



