54 
in-cone structure and stylolites. These are formed probably by 
local pressure. 
Concretionary structure. Clay concretions from many local- 
ities. These take very various and curious forms so that they are 
often mistaken for fossil fruits or parts of animals. They may be 
several feet in diameter, as shown by some of the larger speci- 
mens. 
Septaria. These are formed by the drying and shrinking of 
concretions and subsequent filling of the cracks with calcite. 
When cut and polished, as shown, they are objects of great 
beauty. 
Case 3. — Specimens illustrating other varieties of concre- 
tionary structure, such as geodes, sand and limonite concretions, 
phosphate of lime, chert and flint nodules, oolitic and pisolitic 
limestones, orbicular diorite, etc. 
Specimens illustrating varieties of rock texture, which would 
properly follow here, may be found in Case 9, Hall 66. 
Case 4A.— Stratified structure. This is illustrated by speci- 
mens of South Dakota sandstone, the colors of which bring out 
the strata more clearly, and by specimens of schists and shales. 
Faults, produced by a movement of the rock on a plane inclined 
to that of the strata, are also illustrated here. 
Cellular structure. Illustrated by lavas and scoriae. Where 
the cells have become subsequently filled with mineral matter, 
amygdaloidal structure is produced. 
Case 4B.— Specimens illustrating glacial action. 
Glaciated pebbles and bowlders from the drift of Chicago, 
Rochester, and other localities. Rock surfaces planed and striated 
by glacial movement. Similar specimens from regions of modern 
glaciers, such as Porfirio Diaz glacier, Mexico, and the Mer de 
Glace, Switzerland. 
Case 4:C.— Specimens illustrating effects of erosion by wind 
and water, also stages and forms of rock weathering. Ripple- 
marked surfaces of exceptional size and degree of perfection. 
Surfaces showing mud cracks and rain drops. *‘Slickensided” 
surfaces. 
Case 5.— Large specimens of types already mentioned. They 
include two septaria three feet in diameter, a surface four feet 
