DEPARTMENT OF GEOLOGY 
35 
sedimentary deposits, and fragmental in structure. The prin- 
cipal varieties of these are arranged in this order: Sandstone, 
conglomerate, breccia, quartzite, shale, clay, tufa or tuff, coquina, 
chalk and limestone. 
Then follow metamorphic rocks. These are divided into 
stratified or bedded, and foliated or schistose. The first class 
includes crystalline limestones, marbles and dolomites. They are 
made up chiefly of the mineral calcite, and are formed from the 
remains of molluscs, corals and other animals. These produce 
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 suucture. 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, such as rocks of the copper and iron-bearing regions 
about Lake Superior, rocks of Manhattan Island, lavas of well- 
known volcanoes, including Vesuvius, Mauna Loa, and the ex- 
tinct volcanoes of central France, and lavas and other products 
of the volcanoes of the Valley of Mexico. 
Some large specimens illustrating special rock structures or 
types are shown in the wall cases adjoining the principal collec- 
tion. These include: Specimens from North Wales showing 
slaty cleavage, with leaves six feet long and one-sixteenth of 
an inch thick; various specimens illustrating porphyritic struc- 
ture; a series of specimens illustrating vein structure and 
faults; basalts from ancient volcanic outflows showing joint- 
ing, and a series of dendrites. 
