tones,
schists, and gneisses of ancient age. The talc itself is usually
schistose like the wall rocks, and is largely a product of mechanical
mashing. In some cases, also, talc results from the alteration of
igneous rocks without mashing--as in the case of the large talc and
soapstone deposits of Virginia, which are the result of rather complete
alteration of basic igneous rocks such as peridotites and pyroxenites.

Talc is known to result from the weathering of magnesian silicates under
surface conditions, but the common occurrence of the principal deposits,
in highly crystalline rocks which have undergone extensive deep-seated
metamorphism, is an indication that processes other than weathering have
been effective. It has been suggested that hot ascending solutions have
been responsible for the work, but without much proof. A more plausible
explanation for many deposits is that the talc results from the dynamic
metamorphism or shearing of impure magnesian carbonates (as in highly
magnesian limestones), the process resulting in elimination of the
carbon dioxide and recrystallization of the residue. Certain talc
deposits, such as those of Ontario, show clearly traces of the original
bedding planes of limestone crossing the cleavage of the talc, and the
rock bears all the evidence of having formed in the same manner as a
common slate. Talc and slate are almost the only mineral products which
owe their value principally to dynamic metamorphism.

CHAPTER XIV

EXPLORATION AND DEVELOPMENT

THE GENERAL RELATIONS OF THE GEOLOGIST TO EXPLORATION AND DEVELOPMENT

The economic geologist is more vitally concerned with exploration and
development than with any other phase of his work. This comes closest to
being his special field. Here is a fascinating element of adventure and
chance. Here is the opportunity to converge all his knowledge of geology
and economics to a practical end. The outcome is likely to be definite
one way or the other, thus giving a quantitative measure of the accuracy
of scientific thinking which puts a keen edge on his efforts. It is not
enough merely to present plausible generalizations; scientific
conclusions are followed swiftly either by proof or disproof. With this
check always in mind, the scientist feels the necessity for the most
rigid verification of his data, methods, and principles.

The general success of the application of geology to exploration and
development is indicated by the ra