GEOLOGY OF THE NORTHERN ADIRONDACK REGION 415 



this slip faulting is of the normal or reverse type, that is, 

 whether it took place under conditions of tension or of compres- 

 sion. But the whole aspect of the exposures indicates the 

 action of compressive forces. The best localities for observing 

 the conditions are in the rock cuts along the New York Central 

 and Hudson River Railroad west of Saranac Inn station. All 

 of these show the anorthosite to have been much shattered and 

 sheared, with alteration of the feldspar to saussurite where the 

 action has been most violent. There are four main sets of 

 joints shown in the exposures: a nearly vertical n. 50° w. set, the 

 steadiest and most persistent of all; a nearly vertical n. 80° e. 

 set; a somewhat erratic set, shifting from n. 45° e. to n. 15° e. 

 and not vertical; and a n. 20° w. set, often with a hade as 

 high as 40°. The two latter are the ones which show shearing 

 and slickensiding, and more specially the last one. The shear- 

 ing planes are closely spaced, dividing the rock into very small 

 blocks, and the minerals are much broken and brecciated. Both 

 these sets appear to be compression joints, representing likely 

 two different periods of compression, and the effect of the shear- 

 ing would be much more likely to produce compression faulting 

 than tension faulting. The brecciated rock recalls other brec- 

 ciated strips of the region, notably those described by Kemp 

 from Hammondville, Essex co. 1 These breccias sometimes have 

 a chloritic matrix, but more often one in which quartz and chal- 

 cedony predominate and form hard, resistant rocks, so different 

 from the rather loose masses of fault rock which characterize 

 the Paleozoic fault planes of the region that they would seem 

 unquestionably to be much older. That they mark lines of 

 faulting is certain, and the Precambric age of this faulting would 

 seem beyond dispute. 



The Paleozoic rocks are invariably jointed, but in general 

 but two sets are to be made out, they being vertical and at right 

 angles to each other. They show a somewhat varying direction, 

 but usually the more prominent set has a north-south trend, 

 with the minor one running east-west. They are most irregular 

 and least clean cut in the massive Potsdam and Beekmantown 

 beds. They have approximately the direction of two of the 



^th An. Rep't State Geologist, 1893. p.456. 



