43 
Case 2, — Types of structure. Jointed structure. Illustrated 
by basalt from Mt. Holyoke, Mass. , Auvergne, France, and other 
localities. 
Jointed limestones and conglomerates. In one of the latter 
specimens the joint planes cut quite through the pebbles of the 
conglomerate, showing that the jointing has occurred subsequent 
to the formation of the conglomerate. Specimens illustrating 
cone-in-cone structure and stylolites. These are formed probably 
by local pressure. 
Concretionary structure. Clay concretions from many locali- 
ties. 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 concretion- 
ary structure, such as geodes, sand and limonite concretions, phos- 
phate of lime, chert and flint nodules, oolitic and pisolitic lime- 
stones, orbicular diorite, etc. 
Specimens illustrating variety 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 illustrated here, also in Hall 66, 
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. Bowlder of copper found in Dorr, 
Mich. , which must have been brought across the bed of Lake Mich- 
igan from the Lake Superior copper regions by the continental 
glacier. Rock surfaces planed and striated by glaciabmovement. 
Similar specimens from regions of modem glaciers, such as Por- 
firio Diaz glacier, Mexico, and the Mer de Glace, Switzerland. 
