Cushixg — Potsdam Boundary. 17 



coarse conglomerate, though these also do not agree in color and composition 

 with the basal Potsdam as it usually appears. These loose blocks are mani- 

 festly not far removed from their parent ledge. 



Burke. 



In Burke township is a small and interesting Pre-cambrian outlier, 

 at a distance of over five miles from the main boundary. The rock is 

 well exposed in the Trout river at Mackenzie's mill, half-a-mile north of Burke 

 post-office, and thence may be traced westward for one-third of a mile, the out- 

 crops covering a wedge-shaped area with the apex at the mill, and the base to 

 the west. Both to the east and the west the rock passes beneath heavy drift, 

 concealing its extent in those directions, but Potsdam sandstone crops out near 

 at hand to the north and south. In the former direction and only 100 yards 

 down stream is a twenty-foot cliff of hard, yellowish sandstone, here abruptly 

 cut off. A half-mile to the south, at the village, similar sandstone appears in 

 the stream, dipping in the other direction. The accompanying section shows 

 the observed relations. The sandstone has the lithologic characters which 

 mark the middle and upper portions of the formation, and the writer sees no 

 way of accounting for the structural relations here exhibited except on the 

 assumption that the gneiss is brought up by a pair of faults. 



The exposures here show a red, well- jointed, acid granitic rock composed 

 of quartz and microcline or microperthitic orthoclase and a little magnetite. 

 At the mill two large dikes constitute half the exposure. The southerly one 

 is of syenite porphyry and is 27' wide with the south wall not showing. 

 Thirteen yards north of it is a 15 ; dike of a diabasic rock Avhich differs some- 

 what from the normal diabases of the region and is very coarse grained. A 

 few rods to the westward, in the woods, are two other dikes, both of normal 

 diabase, one of which is noteworthy in that it contains numerous inclusions 

 of the wall rock scattered through it, commonly of small size. Such 

 inclusions are not a common feature in the Adirondack diabases. 



Figure 1. Section north of Burke village 



The occurrence of this Pre-cambrian outlier, probably brought up by 

 faulting, suggests interesting possibilities in the way of other occurrences of 

 like nature, now concealed beneath the drift. 



2 



