53 
HALL 66. 
STRUCTURAL AND DYNA/VUCAL GEOLOGY. 
The specimens here shown, when taken in connection with 
those in Hall 66, illustrate the materials of the earth’s crust and 
the processes of change which have taken place and are still in 
progress upon the earth. They also illustrate many of the more 
striking phenomena met with in the study of the earth’s crust and 
may therefore as well, perhaps, be designated as exemplifying 
Phenomenal Geology. 
Case 1. — The specimens illustrate some results of the action 
of chemical forces. First is placed a series of dendrites — arbor- 
escent impressions on rocks of all kinds, which have often the 
beauty of a drawing. They are produced by the deposition of 
oxides of iron or manganese from waters which have permeated 
the strata. The aborescent forms are due to incipient crystalliza- 
tion similar to that which takes place when frost is deposited 
upon a window pane. Next are shown specimens illustrating 
metamorphism, which may be produced by pressure, as where 
chalk is changed to limestone, or by chemical changes, as where 
pyroxene alters to serpentine. 
Cementation is illustrated by specimens of coquina of various 
types, of trass, and of conglomerates varying in degree of con- 
solidation. 
Then follows an extensive series of cave products, in which 
are shown stalactites and stalagmites from the Mammoth, Luray, 
Wyandotte and other caves, together with gypsum, epsom salts 
and other interesting cave products. Stalactites of sulphur, 
galena, barite and other minerals show that the formation of these 
objects is not confined to limestone regions. 
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- 
