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THE DISTRIBUTION OF ANCIENT VOLCANIC ROCKS.

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also volcanic^[1] rocks are widely distributed through the crystalline belt of eastern North America, and to direct attention to them as offering a new and promising field for work in crystalline geology. For the accomplishment of this purpose it will be necessary (1) to consider the general attitude of geologists in different countries toward ancient volcanic rocks; (2) to specify the criteria available for their identification; and (3) to summarize our present knowledge of where such rocks certainly or probably exist in the eastern crystalline belt. The material embraced under the third of these heads has been obtained from personal work in the field, from a careful study of existing literature, and from unpublished observations and hints furnished by friends.^[2]

It is hoped that the bringing together of what is now known of the distribution of ancient volcanic rocks in eastern North America, with the addition of new areas and indication of localities where they may be looked for, will stimulate further work in widely separated portions of this interesting field. These rocks have, it is true, already been correctly described at a few isolated points, but no attempt has before been made to connect such areas or to show their probably widespread distribution. The recent identification by the writer of a very extensive development of pre-Cambrian lavas and volcanic tuffs and breccias in the South Mountain of southern Pennsylvania and Maryland^[3]

↑ The term volcanic might perhaps be applied with propriety to all rocks produced in or on a volcano, without regard to their structure or coarseness of grain. It is, however, here employed only for effusive or surface igneous rocks, in contrast to such as have solidified beneath the surface, either as the basal portions of volcanoes, or as dykes, sheets, laccolites, or stocks (bathylites).
↑ The writer is especially indebted for help to Professor Eugene Smith, of Alabama; Professor W. S. Bayley, of Waterville, Me.; Professor J. A. Holmes, of North Carolina; Professor H. D. Campbell, of Lexington, Va.; Dr. A. C. R. Selwyn, of Ottawa; Mr. L. V. Pirsson, of New Haven; Professor S. L. Powell, of Newberry, South Carolina, and Mr. Arthur Keith, of Washington. The "Azoic System" of Whitney and Wadsworth, and Professor Van Hise's Correlation Essay on the Algonkian have also proved of much service.
↑ Am. Jour. of Science (3d ser.), Vol. 44, p. 495, Dec., 1892.These rocks have been thoroughly studied by Miss Florence Bascom, whose results may be expected soon to appear in full and adequately illustrated form. See also this Journal, Vol. 1, No. 8, Dec., 1893.

[1] The term volcanic might perhaps be applied with propriety to all rocks produced in or on a volcano, without regard to their structure or coarseness of grain. It is, however, here employed only for effusive or surface igneous rocks, in contrast to such as have solidified beneath the surface, either as the basal portions of volcanoes, or as dykes, sheets, laccolites, or stocks (bathylites).

[2] The writer is especially indebted for help to Professor Eugene Smith, of Alabama; Professor W. S. Bayley, of Waterville, Me.; Professor J. A. Holmes, of North Carolina; Professor H. D. Campbell, of Lexington, Va.; Dr. A. C. R. Selwyn, of Ottawa; Mr. L. V. Pirsson, of New Haven; Professor S. L. Powell, of Newberry, South Carolina, and Mr. Arthur Keith, of Washington. The "Azoic System" of Whitney and Wadsworth, and Professor Van Hise's Correlation Essay on the Algonkian have also proved of much service.

[3] Am. Jour. of Science (3d ser.), Vol. 44, p. 495, Dec., 1892.These rocks have been thoroughly studied by Miss Florence Bascom, whose results may be expected soon to appear in full and adequately illustrated form. See also this Journal, Vol. 1, No. 8, Dec., 1893.

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