34:8 Pirsson and Washington — Geology of N ew Hampshire. 



mass northeast of Hill's Pond. Hitchcock's description also 

 gives clear indications of the same thing in other localities not 

 visited by us. One of the best localities for the study of the 

 contact that was seen by us is at the foot of the west slopes 

 running down from the north end of Piper Mountain in the 

 pasture fields south of Morrill's farm, where the path, by which 

 Mt. G-unstock is generally ascended, begins. The mica 

 schists and other rocks, which we infer make the formation 

 shown by Hitchcock on his map as the Montalban, are full of 

 pegmatite and fine granite stringers and dikelets and Appear to 

 be enriched in feldspar. As the contact is approached they 

 change to a fine dark-gray gneiss which is cut by fine granite 

 dikes. Higher up appears the syenite itself. The attitude 

 and characters of the gneisses are such that they indicate quite 

 clearly that they lie, thinning out toward the mountain, upon a 

 rising slope of the igneous rock below and that the contact 

 plane is therefore here not vertical but dipping away from the 

 mountain. The syenite from the slopes above is that of the 

 main type but finer-grained. At the south end of Piper Moun- 

 tain the bordering granite has a faint but distinct gneissoid 

 appearance. It is also to be noted that these bordering masses 

 of granite are generally filled with spots and streaks of varia- 

 ble size of darker inclusions, which are no doubt fragments of 

 the country rock thoroughly altered by immersion in the 

 magma. 



It appears to us that the best explanation for these fringing 

 granite masses is to consider them a differentiated border 

 facies, an endomorphic contact modification of the main type. 

 They may not exist everywhere, but they have been so gener- 

 ally found on different sides, as seen by ourselves and indicated 

 by Hitchcock, that the phenomenon seems difficult of explana- 

 tion on any other basis. It is true that we have not been able 

 on continuous exposures to trace the gradual merging of the 

 granite into the syenite, because this should be done on the 

 lower slopes, and for reasons given above these do not afford 

 proper exposures for this purpose. We cannot affirm then 

 positively that these are not a series of later eruptions which 

 have broken out around the border of the previously intruded 

 syenite, but in view of their disposition such an explanation 

 seems unnatural, and especially so since they do not exhibit cer- 

 tain phenomena shown by an undoubted later intrusion of 

 granitic magma on the western slope of Locke's Hill, which 

 will be presently described. From the facts at our command, 

 therefore, we are inclined to think the first explanation the 

 more reasonable one and to regard the granite as a differen- 

 tiated border mantle of the syenite. We also do not regard 

 the granite border as having been produced by the melting up 



