214 STRATIGRAPHICAL GEOLOGY. 



of porphyritic gneiss. The stream has cut down several feet into the 

 ledge, and there is something of a fall here. Dykes of trap traverse the 

 gneiss. The dip is south-westerly. Passing up the stream, in the neigh- 

 borhood of a mile, we are brought to "Becky town," the base of the slide, 

 where the great piles of rubbish first become conspicuous in making the 

 ascent. A few rods above is an exposure of the same rocks with those 

 seen at the falls, dipping 80° S. 10° W. The strata are indicated by folia 

 of mica and a little of a dark hypersthenic mineral, often forming nodules. 

 There are jointed planes, also, with a dip westerly of 25°. The feldspar is 

 nodular, the crystals not being so perfect as is common in this formation, 

 or as may be seen upon Cascade brook to the south (p. 103). Between 

 Norway and Cascade brooks there seems to be an anticlinal axis in the 

 porphyritic gneiss. At first I was satisfied that this rock was gneiss, 

 but did not recognize its true place with the porphyritic group. Subse- 

 quently I referred it to the "trachytic" or Albany granite, but a reexami- 

 nation in 1875 shows that it belongs to the oldest of our formations, and 

 is distinctly stratified, traversed by trap dykes and narrow banded veins 

 of quartz. These exposures do not occupy more than two hundred feet 

 of distance. 



A few rods up Norway brook appears the first ledge of the ossipyte. 

 Its junction with the gneiss is concealed by drift. For about a mile 

 similar ledges occur, some exposures being sixty or seventy feet long. 

 Considered as an isolated case, it is difificult to determine the planes of 

 stratification, since two prominent sets of jointed planes exist, either of 

 which might be taken for strata. One set dip about 20° northerly, and 

 are the most numerous. The other dip about 75° westerly. More defi- 

 nitely, the following are the supposed strata dips seen in ascending: 

 about 20° to N. 26° W., N. 86° W., N. 34° E., and N. 46° W. The joints 

 have these strikes, N. 22° W., N., and S. Analyses of both the labradorite 

 and chrysolite, which form the constituents of the ossipyte, have been 

 given in Volume I, pages 37-40, by Hunt and Dana. It appears that 

 small portions of magnetite and biotite arc also present in the rock. 



SlENITE OF TrIPYRAMID. 



The ossipyte is abruptly succeeded by the gray rock called sienitic, 

 being sometimes labradorite, with hornblende and some mica, then ortho- 



