48 
These produced limestone first and this 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 polished 
slabs. 
Upper part of Case 9. — Specimens illustrating rock texture, 
veins, rock folding. A series of wooden models illustrates struc- 
tures produced by faulting. 
Lower part of Case 9 and Case 10.— Following the marbles 
are placed the crystalline schists, which are rocks of variable com- 
position, but characterized by a pronounced schistose structure, 
especially where mica is the prevailing constituent. Here are in- 
cluded argillite, clay-slate, eclogite, quartzite, phy Hite, paragonite 
schist, chlorite schist, mica schist, and others. 
Last in the series appear the gneisses, 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, mus- 
covite, hornblende, or others, are included here. 
DIVISION OF ECONOMIC GEOLOGY. 
It is the purpose of the collections shown in this Division to 
illustrate modes of occurrence in Nature of the minerals and ores 
which have economic importance, to show the localities from 
which they are obtained, the processes used in their extraction 
and treatment, and their application to human arts and indus- 
tries. 
The specimens have for the most part been gathered from ex- 
hibits made in the Department of Mines and Mining of the Col- 
umbian Exposition, and were secured to the Museum by the Chief 
of that Department. 
They may be conveniently classified into five groups, which can 
be most readily inspected in the order named : 
Building stones and quarry products, Halls 67 and 68. 
