In the paper herewith presented the writer has endeavored to illustrate the chemical and physical changes taking place in the breaking down of rock masses through the ordinary conditions of atmospheric action commonly grouped under the name of weathering. The general tendency of the investigations has been the same as those pursued in the case of the disintegrated granitic rocks of the District of Columbia, the results of which were presented to the Geological Society of America at its meeting in Baltimore one year ago.*

Description of Locality.

The rock selected for investigation in the present instance is a coarsely crystalline, somewhat granular diabase, which occurs in the form of a large dike exposed almost continuously from the Mystic river, in Medford, Massachusetts, northward toward Spot pond, for nearly two miles.

Its maximum breadth is stated to be not less than 500 feet, but it narrows gradually northward to . . .

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