Lithology for Museum Purposes. — Gratacap. 285 
relevant and necessary way is desirable. The reiteration of an 
idea in any course of instruction imbeds it more fixedly and 
deeply in the student's thought. The repetition should be ac- 
cordant with itself as a matter of course. 
IV. Lit ho logic Processes. 
The succeeding step to those which have imprinted on the 
mind of the visitor a fairly comprehensive realization of rock 
features, is the treatment of those general processes by which 
rocks, as we know them, have been formed. These processes, 
conveniently for the purpose herein designed to be attained, 
may be considered as three-fold, viz.: vulcanism, inctamor- 
phism, sedimentatio)i, and consolidation, including in this last 
chemical or organic action. • 
Vulcanism is first to be defined, and examples for its two 
formal types given, the pericentric and the regional, including 
under the latter all forms of dykes. As illustrating the former 
such an infallible and luminous example as Vesuvius could be 
chosen with a drawing of sections of its lava flows showing 
superimposition, and an imaginary dissection of its basement. 
The plugged and dissected cone of Kammerbiihl,in Bohemia, 
the pumice cone at Campo Bianco, in the island of Lipari and 
the eroded and reduced Auvergne region are instructive exam- 
ples. Illustrations of regional vulcanism are abundant, and the 
Palisades of New Jersey, Snake river dalles, Obsidian cliff of 
the Yellowstone, Henry mountains are typical. Photographs, 
drawings, can here be wisely, if not too lavishly, used. 
Metamorphic action is less easy to typify and explain. Its 
essential preconceptions are formed in chemistry and crystal- 
lography. It means molecular mobility producing chemical 
compounds and crystalline tissue, under the influence of heat 
and hydro-dynamic agencies. How can such difficult assump- 
tion be popularized? The most intelligible symbols are the 
baking of bread and the vitrification of porcelain, the indura- 
tion of brick with its development of color and the formation 
of refractory kernels of silicates. These are still very approxi- 
mate, are, in a way, misleading. The facts of regional and 
contact metamorphism can be given, hand specimens showing 
metamorphism in slates exhibited and the impression deepened 
that heat and pressure not only bake but crystallize, and that 
