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



the edge of the water and is somewhat altered in consequence. 

 From the inside of a good-sized piece fairly fresh material was 

 obtained for study. 



Megascopic. — Holocrystalline ; dense, and very sparingly 

 porphyritic ; medium gray with a brown tinge and weathering 

 brown ; has a faint, dull, silky luster. Has a few phenocrysts 

 of feldspar which tend to be roughly arranged along one plane 

 in the rock but lie without the individual orientation seen in 

 flow structures. Phenocrysts average about 4 mm by 2 mm by 

 0*5 mm ; tabular ; dull, opaque white. The rock has a rather 

 pronounced schistose fracture. 



Microscopic. — The section shows an interwoven mass of 

 minute feldspar laths with occasional scattered shreds of brown 

 biotite. A considerable amount of alteration of both constit- 

 uents is present, and this, combined with the small size, the 

 laths being about 0*l mm in length, and the general presence of 

 repeated Carlsbad twinning, makes it impossible to say with 

 certainty whether they may not be in part a soda-lime feldspar. 

 It is judged that this is wanting or nearly so, but only a chemi- 

 cal analysis can definitely decide the point, and neither the 

 condition of the material nor the results to be gained would 

 warrant undertaking this. The few phenocrysts are of un- 

 striated alkalic feldspar. The rock is penetrated by tiny vein- 

 lets of secondary quartz mingled with occasional areas of 

 fhiorite. No original quartz could be detected, and a gela- 

 tinization test for nephelite gave negative results. The laths, 

 which often tend to thin plates, lie interwoven without orienta- 

 tion in a typical trachytic texture, except that around the few 

 phenocrysts they are arranged in expansion structure.* 



Classification. — The rock is evidently persalic and perfelic ; 

 it is almost without doubt peralkalic. This would make it 

 nordmarkase. The only question is whether it is sodipotassic 

 or dosodic, whether phlegrose or nordmarkose. Judging by 

 analogy with the other rocks of the region, it is probably the 

 latter. The texture is micro-trachytic ; the few rare pheno- 

 crysts are not sufficient to justify one in calling it a porphyry. 

 It is therefore a trachi-nordmarkase. In the system of Rosen- 

 busch it is a typical bostonite. 



Camptonase ( Camptonite). 



As previously mentioned, there are a considerable number of 

 dark trap-like dikes in and about Red Hill. The microscopical 

 study of these has shown that they should all be referred to 

 the rang of camptonase, or in other words, they are campton- 

 ites. Unfortunately, out of some fourteen different dikes from 



* This Journal, vol. vii, p. 277, 1899. 



