40 NATURE STUDY REVIEW [9 : 2— Feb., 1913 



the presence of water the granites and other igneous rocks may 

 be made to change into gneisses and schists, while the sediments 

 become slates, marbles and schists. The original rock and its 

 metamorphic equivalent are given in the table below : 



Original state. Intermediate form. Metemorphic rock. 



Clay Shal2 Slate 



Sand .Sandstone Quartzite 



Gravel Conglomerate Conglomerate schists 



Calcareous material, shells, Limestone Marble 



etc. 

 All feldspathic rocks Cneisfi 



Ferromagnesian rocks and Schists of various kinds 



other igneous rocks. 



This outline shows at a glance the relation of the metamor- 

 phic rock to the original rock. It must not be thought that there 

 is a distinct dividing line between the original and the metamor- 

 phic rock, for there is not. The one grades insensibly into the 

 other so that all gradations are found. This need not present 

 any difficulty for our purpose. If the features commonly seen 

 in a gneiss can be determined in the specimen in hand, even 

 thou,s;h faintly, call it a gneiss. This is satisfactory for a field 

 classification such as the one given here. If the gneissic features 

 are developed, as noted above, and the minerals are those of a 

 grani'te, the rock may be called a gneissic granite. 



The distinctive features of gneisses and schists are the ar- 

 rangement of the minerals into more or less parallel bands and 

 the development of a fairly good cleavage in the rock. (See 

 figures o and G.) This cleavage is due to the fact that minerals 

 with good cleavage are developed during the process of metamor- 

 nhism. The micas with their perfect basal cleavage and the 

 long slender hornblende crystals with their good cleavage are 

 the most common of these minerals. Gneisses always contain 

 feldspar and are nearly always more coarsely blended than the 

 schists. This fact should be noted in determining a gneiss. The 

 schists very commonly show a great deal of mica on the cleavage 

 plane and are often very thinly banded, as thin as a sheet of 

 paper or thinner. (See fieure G.) The bands in both rocks are 

 often bent and folded. (Figure G.) Garnets and crystals of 

 magnetite very often occur in gneisses and schists. These rocks 

 are very commonly found in reo^ions where mountains have been 

 formed, hence we find them from Maine to Georgia and Ala- 

 bama, and in a great many localities in the western part of the 

 United States. In areas where the very oldest rocks we know 

 are found there are always a great many metamorphic rocks as 

 in Wisconsin, Michigan. Minnesota and many other states. 



