18<*.J Oil [Branner. 



The Maucli Chunk red shales are wanting about the north-east end of 

 the basin, but they form thick beds in the Shickshinny end. When un- 

 covered and exposed to the action of air, water and frost, these rocks have 

 gone to pieces rapidly, but they have preserved the striation remarkably 

 well wherever they have been covered up by a considerable thickness of 

 earth. Striation on these shales therefore, as indeed upon most of the 

 rocks, is only found along the roads and cuts where they have been re- 

 cently uncovered. 



The Pocono sandstones, forming the crests of the mountains on both 

 sides of the valley, lie, for the most part, in a desolate, uncultivated, un- 

 inhabited and untraveled region, in which but few striated exposures are 

 to be found. This is particularly unfortunate, for we must, of course, look 

 to the markings upon these high points for the indications of the direction 

 of the ice sheet when it had attained its grandest proportions, and before 

 its margin was here reduced to the condition of local glaciers. This for- 

 mation is the limit of my observations on the glaciation of this region. 



Striation ; its Indications of Flow, Change and Wear. 



Scarcely a place can be found in the valley, which, if the rocks have 

 been protected from the weather by a covering of earth, does not retain 

 some signs of wearing by ice. Where the rocks have been long ex- 

 posed to the action of air and water the well denned lines have, for the 

 most part, been defaced. But even in these cases, the rounded faces of 

 the rocks are often still preserved. But though the striae are, in all 

 probability, well preserved over almost the whole of this region, the 

 drift and soil, covered, for the most part, with forest and undergrowth, 

 render it impossible to make the record as complete as desirable. Most of 

 the observations made upon the direction of the striae have been placed 

 upon the accompanying maps, and it is unnecessary to speak of them in de- 

 tail. Some of the observations have been omitted in cases where several 

 similar ones were made too near each other to warrant drawing several 

 arrows upon the map parallel to the first one. Where there are two or 



markings cannot be detected. I found that by gently rubbing a hard (6 H) pencil 

 across the worn surfaces until they were quite covered with the lead, the fine 

 strise would stand out as white lines. Mr. George M. Lehman of the Survey, 

 who has rendered me valuable assistance by noting the striation in places that I 

 did not visit personally, also found that on a large polished surface, the lines 

 could be detected by the observer taking such a position that the sun would be 

 reflected from it to his eyes. It is necessary in this case, however, that the plane 

 of incidence and reflection should be parallel with the direction of the stria?. 

 I have found this method a useful one, though it is open to the objection that 

 one cannot always have the sun in the desired position. This difficulty may be 

 obviated by making the observations at night, and using a lantern for the re- 

 flections. When good exposures, sufficiently close to each other, can be had, it 

 is not necessary that so much pains be taken to get an observation, but it not 

 infrequently happens that it is very desirable to have one in some particular 

 spot, and where the nature of the rook and the strise require some such methods 

 as the ones mentioned. 



