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an occasional ingredient in the gneiss, but in great dykes or veins 

 penetrating tiie strata. It may be stated as a general feature in 

 the geology of this region, that mica, talc, chlorite and other 

 laminated minerals of the micaceous order — prominent ingredients 

 in the more schistose primary strata — rarely prevail to any extent 

 as regular constituents of* the gneiss rocks of the Highlands. 

 In this respect this whole primary chain, viewing it from the 

 Delaware to the Hudson, presents a striking contrast to the other 

 zone of primary stratified rocks, which traverses the country 

 nearer to the seaboard with little interruption from New England 

 to the Southern States. The gneiss rocks of that belt bordering 

 Long Island Sound, passing through New York and Staten 

 Island, reappearing at Trenton, and ranging through Pennsyl- 

 vania and Maryland, are distinguished for the prevalence of mica 

 and other thinly laminated minerals, imparting to them either the 

 schistose structure or the more or less thinly-bedded character of 

 ordinary gneiss. 



A common feature in the massive gneiss of the Highlands is, 

 a tendency to parallelism in the arrangement of its minerals, 

 especially of the felspar and hornblende. In this case the crystals 

 are of a flattish form, and are apt to lie in thin and somewhat 

 separate alternate layers in the rock. This structure seems strictly 

 in harmony with the doctrine which assumes that the so-called 

 primary stratified rocks have been once sedimentary deposits, 

 like the secondary strata, modified into their present crystalline 

 texture by a heat approaching to a partial fusion of the materials. 

 The relative absence of mica and of the more thinly laminated 

 or schistose character, so predominant in some portions of our 

 southeastern primary belt, has arisen, if this hypothesis, usually 

 denominated the " metamorphic theory," be correct, simply from 

 a relative deficiency in the original deposits of those earthy mat- 

 ters, such as clay, lime, magnesia, &c-, which are the ingredients 

 of mica and the minerals most nearly allied to it; or, what is the 

 same thing, from a relative excess of silica and those other earths 

 or oxides which are constituents of quartz, felspar, and hornblende. 

 The influence of a difference of temperature in bringing about a 

 diflerence in the mineral aggregation of the various earths, pro- 

 miscuously mingled at the period of their deposition, is also 

 possibly connected with the marked contrasts which we see 



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