6o 
mals, producing first limestone, which was changed by the 
action of heat to the crystalline condition. In some cases the 
original fossils remain intact, as is illustrated in many of the pol- 
ished slabs. 
Upper part of Case 9.— A large and complete collection of 
varieties of 7narble, the different colorings being produced largely 
by iron oxides, micaceous minerals, or finely distributed sediment. 
Lower part of Case 9 and Case 19. — Following the 
77iarbles are placed the crystalline schists^ which are rocks of var- 
iable composition, but characterized by a pronounced schistose 
structure, especially where mica is the prevailing constituent. 
Here are included argillite, clay-slates, eclogite, quartzite , phy Hite , 
parag07iite schist, chlorite schist, 77iica schist, and others. 
Last in the series appear the g7ieisses, a class of rocks essen- 
tially like the granites in composition, but differing from them in 
structure, in that the constituents are arranged in approximately 
parallel bands or layers. These are the oldest of crystalline rocks, 
and are considered by many to represent portions of the primeval 
crust. Others, however, regard granites as the last term in the 
metamorphism of such rocks, and for that reason the gneisses 
have been placed in juxtaposition to them. Varieties of gneiss, 
based upon the prevailing mineral, whether biotite, 77iuscovite, 
hor7ible7ide, or others, are included here. 
