No. 10-i.J 181 



crops and map the surface according to the several prevailing rock 

 types. Another season it is proposed to traverse the belt on numerous 

 section lines and then follow the boundaries of these unstratified areas 

 and determine their relations to the surrounding rocks. Their exist- 

 ence in the Highlands east of the Hudson is analogous to the results 

 of the survey3 and studies made in the south-western continuation of 

 the same belt in New Jersey.* The fact of detached or isolated areas 

 of rocks, marked by certain lithological characters in the broad region of 

 crystalline rocks of western Connecticut and south-eastern New York 

 was announced in 1842 by Dr. Percivalin his Geology of Gonnecticut.\ 

 It has been suggested as true of the crystalline rock of New Hamp- 

 shire; also, where there are 23 of these ovoidalare as of Laurentian rocks.| 



In describing the outlines of the Highlands Archa3an rocks, refer- 

 ences to their general characters have been incidentally made. The 

 kinds of rock most common are gneiss, syenite gneiss, granite, quartz- 

 syenite, granulyte and hornblende schist. The varieties under these 

 heads are very numerous, since the constituent minerals are present in 

 so varying proportions. Orthoclase and quartz prevail, but mica (usu- 

 ally biotite) is rarely altogether wanting. Hornblende, also, is com- 

 mon. Triclinic feldspars, muscovite and augite occur frequently, and 

 of rarer occurrence and as accessory constituents in the prevailing 

 rock types are epidote, graphite, magnetite, apatite, ilmenite and 

 pyrite. These latter minerals in places make up so much of the mass 

 as to give names to the rock ; and there are magnetic iron ore, tita- 

 niferous ores, beds of pyritiferous rocks, of graphitic gneiss and of epi- 

 dotic gneiss. 



The more common rocks are rather coarse crystalline and the con- 

 stituent minerals are readily identified by the aid of a good lens. The 

 tine crystalline to amorphous varieties occur within very limited out- 

 crops usually, and are quite subordinate in importance. The feldspars 

 are generally prominent in the crystalline mass of the rock, often giv- 

 ing it a porphyritic aspect. And in the coarser crystalline varieties 

 orthoclase clearage surfaces are in some cases an inch in diameter. A 

 common phase of the prevailing gneiss is a rude parallelism in the 

 mineral arrangement of the rock mass. This aspect may be due to 

 simple alternations of thin layers of the feldspars and the quartz, but 

 it is more generally observed in the case of the micaceous and micaceo- 

 hornblendic varieties, where the dark lines of the biotite and horn- 

 blende are in contrast with the Avhite to flesh-colored feldspathic por- 

 tions. Where the mica is abundant, the rock assumes a schistose 

 structure. 



Viewed as to chemical composition the feldspathic, or granulyte- 

 like rocks are more siliceous and contain more potash and soda ; the 

 hornblendic and biotite gneisses are characterized by less silica and less 

 of the alkalies, but more iron. § And the latter occur usually near the 

 beds of iron ore or are associated with them. Some authors have con- 

 sidered the former as an acidic or highly silicated class and the latter 

 as basic or poorly silicated. The query is here propounded that, 

 perhaps, in these marked difEerences in composition there may be a 



■^ Ann. A'ep.<. of State Geologist of Miu Jersey for 1884, pp. 05-67 ; and for 1885, pp. 



^ Report onthe Geology of C'onnecticut, by J. G. Percival, 1842, pp. 143-144. 



j Descriptions of Geological Sections across New Hampshire and Vermont, by Prof. Chas. 

 E. Hitchcock, State Geolorjist of New Hampshire, 1884, pp, 29-30. 



§M. de Lapparent in his Trait'e, de Geotogie, 1885, refers to the division of the Archiiean 

 system into a lower series (etage) marked by the presence of silica and alkalies, and by 

 greater uniformity of cliemical composition and an upper series whose rooks are more di- 

 verse in composition and contain more iron, lime and magnesia. 



