DEPARTMENT OF GEOLOGY 
39 
limestone first and this is changed by the action of heat to the 
crystalline condition. In some cases the original fossils remain 
intact, as illustrated in many of the polished slabs. Following 
the marbles are placed the crystalline schists, which are rocks of 
variable composition, but characterized by a pronounced schis- 
tose structure, especially where mica is the prevailing constit- 
uent. Here are included argillite, clay-slate, eclogite, quartzite, 
phyllite, 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 approxi- 
mately parallel bands or layers. Varieties shown depend upon 
the prevailing mineral. 
In addition to the systematic collection above described 
some special collections, illustrating rocks of certain areas, are 
shown as follows: 
Rocks of the copper and iron-bearing regions about Lake 
Superior. 
Rocks of Manhattan Island. These were obtained chiefly 
from excavations made in and about New York City. They are 
crystalline, metamorphic rocks, and illustrate the great varia- 
tions possible in kinds of rocks in a small region. 
Rocks of the Green Mountain Range, as seen in passing east- 
ward from Pittsfield, Massachusetts. These include a variety 
of schists, limestones, and other metamorphic rocks ranging in 
geological time from the Archaean into the Devonian. They 
illustrate the different formations distinguished by geologists 
in the region. 
Lavas of well-known volcanoes, including Vesuvius, Mauna 
Loa, and the extinct volcanoes of central France. 
Lavas and other products of the volcanoes of the Valley of 
Mexico. 
Halls 76 and 77, 
GEOGRAPHIC GEOLOGY. 
The purpose of this collection is to illustrate in a vivid and 
realistic way the surface configuration of the earth. The chief 
feature of the exhibit is a series of relief maps which reproduce 
on as natural and representative scales as practicable, the topo- 
graphy and structure of selected portions of the earth’s surface. 
A part of the series shows only topography and sculpture, while 
