264 



PROCEEDINGS OF THE ACADEMY OF 



[1880. 



unaltered rock, except possibly in the 

 vicinity of streams where the water has 

 eroded away the soft rock. In such posi- 

 tions, however, the gravel also is usually 

 eroded, except in the case of the " River 

 gravel " proper. It will be observed that 

 a sharp stratified micaceous sand, made up 

 of the materials of the decomposed gneiss, 

 and often showing " flow and plunge " 

 structure, lies below the gravel. In the 

 section given, a well-rounded boulder of a 

 lower Silurian sandstone is seen partially 

 imbedded in the decomposed gneiss. This 

 fact offers two interesting deductions : — 



(1.) That the gneiss was decomposed 

 before the deposition of the gravel. 



(2.) That water, not ice, was the agent 

 of such deposition. 



(1.) As additional evidence in support 

 of the first deduction, it has been observed 

 in several sections that portions of the de- 

 composed gneiss have been taken up and 

 interstratified in horizontal layers, either 

 with the gneissic sand, or with the gravel 

 itself That the steeply-dipping decom- 

 posed gneiss should be thus re-stratified, as 

 though by a flood, and that, on the other 

 hand, no such phenomena are ever observed 

 in undoubted glaciated regions, can only be 

 explained upon the assumption that the 

 gneiss was decomposed before the Glacial 

 epoch. That such decomposition took 

 place in a j^et earlier geological age, will be 

 indicated below under a description of the 

 " Bryn Mawr gravel." 



(2.) Absence of a glacier in this region 

 is indicated by the wave-like junction of 

 gravel and clay, by the stratification of the 

 gravel, and by the presence of decomposed 

 gneiss. No polished surfaces of rock have 



Fig. 3. 



1+ 



