THE DISTRIBUTION OF ANCIENT VOLCANIC ROCKS. 3 



also volcanic^ 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 geol- 

 ogy. For the accomplishment of this purpose it will be neces- 

 sary ( I ) to consider the general attitude of geologists in differ- 

 ent 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 from unpublished observations and hints furnished by friends.^ 



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 locali- 

 ties 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 devel- 

 opment of pre-Cambrian lavas and volcanic tuffs and breccias in 

 the South Mountain of southern Pennsylvania and Marylands 



^ The term volcanic might perhaps be applied with propriety to all rocks pro- 

 duced 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 Ala- 

 bama ; 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 Algon- 

 kian 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. i. No. 8, 

 Dec, 1893. 



