212 MINERALOGY AND LITHOLOGV. 



plastic under the influence of great pressures, which are partially due to 

 the weight of superincumbent strata and partially to lateral pressures, 

 then, in the disturbances which occur in the course of the elevations of 

 mountains, the lateral pressures are the ones which naturally are first 

 relieved by fractures of the earth's crust; and impeded movements, 

 therefore, are very liable to take place in the plane of the strata, and all 

 the conditions are fulfilled for the formation of gneisses, with the plane 

 of lamination parallel to the strata of the region: this is the position 

 which the laminae of most gneisses occupy. All these suppositions are 

 quite old, as will be noticed, yet they have received but little attention in 

 this country. They are also applicable in the study of the structure of 

 other crystalline schists ; and in general, though the influence of original 

 sedimentation upon rock structure is not to be lost sight of, and although 

 rocks of this class are generally considered to have resulted from the 

 recrystallization of stratified sediments, it seems certain that the present 

 condition of all the crystalline schists is largely owing to the effects of 

 the pressures to which we know they have been submitted. 



At the time the last report upon our geology was made (in 1844), the 

 metamorphism of the crystalline schists was generally supposed to be 

 due to the heat which was communicated to the strata by fiery masses 

 of molten material which was erupted through them. In a small dyke 

 of trap or granite, cause sufficient was seen to spread metamorphism far 

 and wide through the surrounding strata. This idea is not yet entirely 

 passed away, although the real effects, which can be produced in sedi- 

 ments by the influence of immense eruptive masses by simple contact, 

 are evidently small, as can be seen in many places, — for example, in the 

 Connecticut valley, where the great dykes of diabase cut the red sand- 

 stones. The microscope points to the action of the same forces in the 

 recrystallization of the schists as in the case of the granites, though 

 acting perhaps less efficiently in their fusion. 



Muscovite Gneiss. Pure muscovite gneiss, like granite, is compara- 

 tively rare, because the entire removal of iron from sediments is usually 

 attended by the removal of so many other constituents as to produce the 

 composition of some other rock. A bed of pure muscovite gneiss occurs 

 at Chesterfield. The rock is white, glistens with its mica, and contains 

 many light red, poorly crystallized garnets. The mica lies in very reg- 



