GEOLOGY OF MOUNT MARCY 5/ 



include a huge slab of Grenville gneiss. Two strongly marked faults 

 are exposed with crushed and decomposed rock between well-defined 

 walls. The master-fault strikes N. 67° W. and has a dip varying 

 from vertical to 55° south. A smaller fault strikes N, 57° W. and 

 is nearly vertical. A minor sheeting is also developed in a northeast 

 direction. The first-named fault runs very true with the general 

 course of the valley and pass containing Chapel pond. Although 

 now at one side of the stream it was probably influential in directing 

 the original trend of the valley. The very marked northwest and 

 southeast courses of the minor brooks give good ground for infer- 

 ring many other lines of crushing and weakness in this direction, 

 and the pinched and sheeted rock exposed in numerous cascades 

 corroborates their existence ; but the great structural breaks are north- 

 east and southwest. 



The very peculiar and right-angled relations of the drainage lines 

 led Prof. A. P. Brigham, from a study of the maps of the Mount 

 Marcy quadrangle and its neighboring ones to the east, southeast 

 and south, to describe them and give the name " trellised drainage " 

 to them.^ The name is appropriate and descriptive. This particular 

 area furnishes its best illustration. The bedrock projects so gen- 

 erally in the higher mountains that the mantle of glacial drift has 

 not sufficed to divert the drainage from the lines of structural weak- 

 ness and superimpose a new system, not dependent upon them. 



While the writer js convinced that these peculiarities of the 

 drainage are primarily due to fault-lines, and that a great 

 number of other faults exist, than those plotted on the map ; yet 

 only those have been indicated by a special symbol which seemed 

 so well demonstrated by crushed zones and by pronounced escarp- 

 ments as to leave little room for doubt in the mind of an observer. 

 The northeasterly lines have been so often occupied by the basaltic 

 dikes, which are regarded as Precambrian, that the faults themselves, 

 if this assumption of the age of the dikes is true, must be still older 

 than the dikes. 



Areal distribution of the several formations. The Grenville 

 is chiefly in the northeastern border of the quadrangle. It is best 

 developed on both sides of the East branch, but is greatly cut up 

 by the intrusives. Some prospecting pits have disclosed the lime- 

 stones west of Owl's Head peak. There is also the included lime- 

 stone southeast of the Cascade lakes. If the interpretation of the 



1 Brigham, A. P., " Note on TrelliseH Drainage in the Adirondacks," Amer. 

 Geol., 21 :2i9, 1898. 



