MUSEUM OF COMPARATIVE ZOOLOGY, 329 



channel farther south than our map extends. If this view is correct, it 

 follows that Big Falls, like so many other cascades in New York and 

 other glaciated States, result from an old stream's taking a new course 

 (I believe this to be a general explanation for many falls) ; that the 

 Kaaterskill now makes its way through tlie Little Mountains over -a 

 broadly open pass that once led from the Hudson to the Marcellus 

 valley ; that the Catskill, although larger than the Ktmterskill, was 

 accidentally turned into a smaller cross-valley, which it has since deep- 

 ened somewhat into the appearance of a gorge. Such distui'bance of 

 pre-glacial drainage is a very common occurrence in our Northern regions, 

 and often gives rise to lakes. 



Further study is needed to leam the features of the Helderberg 

 folds as they extend northward to the south line of Albany County, 

 where Mather (1843, 335) says they end, and to show their increase 

 to the south. 



Since writing my article for Appalachia, several months ago, an excur- 

 sion to the mountains in Central Pennsylvania has given me new reason 

 to repeat what was then said concerning the advantages of the Little 

 Mountains for studying the elements of topography ; for in Pennsyl- 

 vania the structural features ai-e on so much vaster a scale, that days 

 are there required for what can be seen much more clearly in a few hours 

 near Catskill. 



This article is accompanied by two plates, numbered XIL and XIIL 



Plate XII. contains sixteen figures, described in the text. 



Plate XIIL is a colored map showing the district here described. 



Cambridge, January 15, 1883. 



