212 MINERALOGY AND LITHOLOGY. 
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 laminz 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 zmmense 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- 
