Cushing — Potsdam Boundary. 



21 



granitoid gneiss of the region. These relations are well shown one-half mile 

 north-west, and again the same distance south of the Centre. At the latter 

 locality a hornblende gneiss at the base of the section grades upward into 

 gabbro-diorite and this in turn into red orthoclase-quartz gneiss. A 

 considerable biotite content characterizes some of the omeisses here and is 



O 



worthy of note as it is not an important mineral inmost of the basal gneisses. 



The gradual passage of one kind of gneiss into another in this region is 

 not thought to possess the significance which would attach to it in an 

 unmetamorphosed district. It is so general, and the rocks concerned are often 

 so diverse that it would seem to have been produced during the metamorphism 

 of the region and therefore to be secondary, instead of representing an original 

 structure due to community of origin. North-west of Dickinson Centre quite 

 massive gabbro-diorite is seen passing over into red orthoclase gneiss, not 

 however by a gradual change but by the most minute kind of interbanding of 

 the black and red gneisses, both of which here have a composition inter- 

 mediate between that of the two extreme varieties. 



One mile west from the Centre on Macomber's farm, is a ledge of Potsdam 

 sandstone outcropping one- fourth mile south of the road. It is red in 

 color and thin bedded, but hard and firm and lias been quarried 

 somewhat for local use. It has the usual moderate dip to the north- 

 west. To the south it shows cut off edges, a marshy tract intervenes, then at 

 a distance of 75 yards appears a massive wall of red, acid, microcline-quartz 

 gneiss. This kind of topography prevails where the gneiss and the sand 

 stone are found near together and may perhaps be accounted for by the easily 

 erodable character of most of the basal Potsdam. 



East and west of Dickinson Centre along the Deer river are heavy sand 

 deposits which cover considerable territory. They are on nearly the same 

 level as the upper sand at Malone, but probably represent deposits along 

 stream made by the river during its flooded condition following the retreat 

 of the ice sheet, while local glaciers may have still lingered in the Adirondacks. 



ST. LAWRENCE COUNTY. 

 Hopkinton. 



The boundary pursues a nearly east and west course across this 

 township. . It presents certain differences in character when compared with 

 the boundary in Franklin county, owing to the fact that the gneisses here strike 

 east and west so that the ridges run parallel to the boundary instead of coming 

 up to it, while depressions are less frequent and cut across the strike. Out- 



