264 Pirsson and Washington — Geology of Red Hill, N. H. 



sense, paisanite, syenite porphyry and for the traps camptonite. 

 They occur in the syenite itself as noted by previous observers 

 and in the surrounding gneiss, and sometimes at long dis- 

 tances, from the mountain. The following list covers the 

 chief occurrences of salic dikes observed. 



The rock in the quarry in the field opposite the Home 

 farmhouse is cut by an aplitic dike about six inches wide of a 

 light gray. It is an arfvedsonitic liparase or paisanite, de- 

 scribed later. Trend E. and W. 



A block in the stone wall at this locality is from a dike of 

 syenite porphyry, dark gray with feldspar phenocrysts. A 

 gelatinization test with nitric acid shows that it is a nephelite- 

 syenite porphyry. 



Another dike of syenite porphyry cuts the granite gneiss of 

 Fore Point on Lake Asquam opposite the north end of Red 

 Hill. The dike is about 10 feet wide and is exposed in the 

 ledge only a short distance. The rock is much crackled and 

 jointed and weathered to a brownish gray. It is line-grained, 

 dense in texture, thickly dotted with small feldspar pheno- 

 crysts. 



Fragments of a similar porphyry with less abundant pheno- 

 crysts and weathered to a red-brown color occur in the debris 

 on the top of North Peak, indicating a dike or dikes of this 

 rock, but the outcrops themselves could not be found. 



On the shore of Hoag's Island in Lake Asquam, beyond the 

 western boundary of the map, there is a dike of a weathered 

 grayish-brown felsite (bostonite). It is three feet in width, 

 cuts the gueiss with a trend to Red Hill and has a somewhat 

 schistose or platy fracture. 



Femic Dikes. — Rocks of lamprophyric character, heavy, 

 dark, basaltic or trap-like are much more numerous than the 

 lighter colored felsitic ones mentioned above. Their occur- 

 rence at the contact at the eastern foot of Red Hill has been 

 already mentioned. Their presence in the nephelite syenite 

 has been already alluded to by geologists who have previously 

 visited the mountain, as detailed in the historical summary, 

 but the locations are described in so vague a way that it has 

 been found impossible to identify them with any certainty. 

 The following have been noted by the writer in traversing the 

 region, though no special attempt has been made to hunt them 

 up and map them. Since the region in general is heavily cov- 

 ered with glacial drift, it is safe to say that for each one which 

 has been found there are dozens which have not been seen. 



Just above the Home farmhouse the syenite exposed 

 in the roadway is cut by a dike, four feet wide, of a black 

 rock, weathering brown, dotted with rather frequent dull white 

 feldspar phenocrysts and small amygduies of calcite. 



