36 FIELD MUSEUM OF NATURAL HISTORY 
also cross ripples. Large surfaces of petrified mud cracks are 
also shown. In the next case the different results of glacial 
action are chiefly illustrated. The series includes glaciated peb- 
bles and boulders from glacial regions of Switzerland and Mexico, 
also from Chicago and Rochester, New York. Among the most 
interesting of the specimens are a series of boulders of copper 
found for the most part about the southern end of Lake Michigan 
which were brought by glacial movement from the copper-bear- 
ing regions about Lake Superior. The largest of these boulders 
weighs 375 pounds. Rock surfaces, planed and striated by gla- 
cial movement, are also shown. These are from both ancient 
and modern glaciers. Specimens illustrating the effect of ero- 
sion by wind and water, also stages and forms of rock weather- 
ing are shown in this case. Numerous specimens of sandstone 
from the Black Hills with differently colored strata are shown. 
Cellular structure is illustrated by lavas and scoriae. In the 
next case jointed structure is illustrated by basalts from Mt. 
Holyoke, Massachusetts, Auvergne, France, and other locali- 
ties. Vein structure and faulting are also fully illustrated. The 
next case is largely devoted to concretions. These include 
lime concretions or clay-stones of many forms and from many 
localities; also sand, limonite and other concretions. A related 
structure called orbicular, found in eruptive rocks is illustrated 
by large slabs from California, North Carolina, and Sweden. A 
number of geodes are shown illustrating the forms and size of 
these bodies. The next case is largely devoted to septaria. 
These are formed from concretions by drying and shrinking and 
a subsequent filling of the cracks with some other substance. 
Several of those shown are of large size, one being nearly three 
feet in diameter. Polished sections are also shown. The next 
case illustrates cave formations and cave life. Stalactites and 
stalagmites and other cave formations chiefly from Indiana 
caves are mounted in natural positions so as to illustrate the 
formation and characters of typical limestone caves. The forms 
of life inhabiting such caves are also shown. The next case is 
devoted to specimens of individual stalactites and stalagmites 
of large size or perfection of form; also two fulgurites, one of 
these being of exceptional size. Adjoining this case a large 
specimen of lodestone from Missouri is shown. On the floor of 
the hall is a collection of basalt columns from the Giant’s Cause- 
way, Ireland, and the Rhine valley. These are grouped so as to 
