AMERICAN GEOLOGISTS AND NATURALISTS. 73 



fleeting mountain wall. High on the side of the ridge, the strige 

 ascend the slope obliquely, but nearer the base they are parallel 

 to the medial axes of the valley. Near the lateral notch, in the 

 northern mountain at Nanticoke, they point toumrd the gorge, 

 showing that a portion of the current here came from a quarter 

 south of east. A great northern wave would, so long as it sub- 

 mersed in its first impetuous rush the summits of the mountains, 

 move forward regardless of the local inequalities of the surface, but 

 after it had partially subsided, the long parallel ridges would pre- 

 sent so many barriers to divide and locally deflect the now feeble 

 remnant of the drainage. Reviewing the phenomena which he 

 has observed. Prof. Rogers concludes that the stria? were pro- 

 duced by the friction of the overlying stratum of drift itself, urged 

 into rapid motion from the north by one or more sudden inunda- 

 tions. From the absence near the southern border of the striated 

 region of granitic, or other far transported northern bowlders, he 

 infers that floating ice, while it may have been concerned in dis- 

 persing the detrital matter from the north, has had no agency in 

 furrowing and smoothing the surfaces of the strata. 



The same genfleman next adverted to the origin of conglom- 

 erates and other coarse mechanical strata, attributing them in 

 many instances to the similar agency of the sea-wave produced 

 by earthquakes. The wide and uniform distribution of some of 

 the coarser rocks of the Appalachian basin, was appealed to in 

 proof that they could have been spread out as we find them only 

 by a sheet of water as broad as the entire margin of an ocean, 

 brealdng in successive sea-waves upon the land, and abrading 

 and dispersing the fragmentary matter during repeated oscilla- 

 tions of the crust. 



Prof. Rogers then added some remarks respecting grooved 

 and polished surfaces at the contact of ancient secondary strata. 

 He thinks he has seen unequivocal instances of these in Penn- 

 sylvania. Their production at periods when the earth's tempera- 

 ture was manifestly incompatible with the existence of ice, would 

 seem to demonstrate that angular detrital matter, urged by water, 

 is able of itself to score and polish the surfaces of rocks. 



Prof. W. B. Rogers continued the illustration of this subject, 

 by calling attention particularly to the evidences of ancient denu- 

 dation and drifting action, so strikingly displayed along the place 

 of junction of the Oriskany sandstone, (Formation VH, of the 

 Pa. and Va. Reports,) and the subjacent limestones, (Formation 

 VI.) In many districts the limestone has been irregularly denu- 

 ded, and even to a gi-eat extent removed, and at the same time 

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