164 STRATIGKAPIIICAL GEOLOGY. 



the spotted granite. Both to the north and south of the stream, it forms 

 a comparatively broad belt. The slates first seen are somewhat indu- 

 rated, and dip 55° N. 62° W. A little higher they stand on their edges, 

 and presently the dip is 55° N. 80'' W. Here we left the valley and 

 climbed up a steep slope, bare of vegetation, because removed by pow- 

 erful currents of v/ater in the time of freshets. Ledges of slate, having 

 about the same position with those just described, occur frequently at the 

 steeper part of this ascent. More particularly, about one hundred and 

 eighty feet up, the rock is an argillaceous quartzite, dipping 75° W. It 

 is well exposed for several hu.ndred feet vertically with the same position. 

 Above that it is less siliceous and more argillaceous, and slightly wrinkled. 

 The dip is often 70° or more, and again 60° N. 82° W. At various places 

 below and above we found slates rusty from an external ferruginous coat- 

 ing, with the interior whitish. The top of Mt. Tom is broad; and you 

 may travel a considerable distance over a slightly inclined surface, where 

 the trees are a foot or more in diameter, even to the very summit. At 

 the saddle, between Tom and Field, the spotted granite is in place, but at 

 the highest point the rock is a feldspathic indurated slate. 



I have descended the mountain easterly three times, by somewhat 

 different routes. The rusty, felsitic slates, first seen in going down, 

 become white when broken. Passing northerly we meet with large 

 blocks of spotted granite, not certainly in place, but it has little thick- 

 ness, and then we find ordinary argillo-mica slates ; and, under them, 

 others charged with andalusite. The dip is very high in the direction 

 N. 82° W. 



A matter of interest may be seen on top of a north-easterly spur of 

 the mountain which lies upon the east side of a stream flowing northerly 

 at first, and joining the south branch of the Ammonoosuc about two 

 miles below the Fabyan house. The rock is a black slate, crowded with 

 pencils of white andalusite, being the same layer with that yielding the 

 numerous boulders of this nature so conspicuous in the streams. The 

 dip is 60° N. 72° W. If this knob deserves a name, it should be called 

 Mt. Andalusite, after the beautiful crystals first found here by Messrs. 

 Abbott and Bacheler, lost sight of for some time, and then rediscovered 

 in 1873. The position all over it is essentially as just indicated. We 

 found the same rock to the south-east, trending towards Cascade brook. 



