ERUPTIVE CHARACTER OF THE ROCKS 369 



a thin-section of one of the rocks showing this structure from the 

 Negaunee district, which has its analogue in several sections of the Vir- 

 gilina rocks. 



The minerals composing the rocks, which are chiefly secondary, are 

 those which would result from chemical and structural metamorphism 

 of an original igneous rock of basic or intermediate composition, 



While, as already stated, in most instances all trace of the original 

 minerals in the rocks is lost or destroyed, in some sections enough re- 

 mains to tell with some degree of certainty what their original essential 

 minerals were. 



The analyses of these rocks given in the table on page — confirm their 

 igneous character. When compared with similar analyses of well known 

 igneous rocks of a certain type from widely separated localities, fairly 

 close agreement is shown in the essential chemical features. Knowing, 

 therefore, the greatly altered condition of the bulk of the rocks in the 

 area, such differences as are brought out in the table of analyses are 

 readily explained on the basis of chemical and physical metamorphism. 



CHEMICAL EVIDENCE 



The chemical analyses of these rocks have been previously discussed 

 in this paper — pages 363-367. The close conformity in composition of 

 the rocks (the least altered ones), as there indicated, with that of andesites 

 from well known but widely separated localities is certainly indicative 

 of igneous origin. Their uniform composition is in contrast with that 

 of a series of clastic rocks, where, as shown by Rosenbusch,* the chem- 

 ical proportions are largely accidental. The microscopical study fully 

 confirms the chemical evidence favoring the igneous origin of these rocks. 



Comparison with other Areas 



Scattered areas of ancient volcanic rocks have been recognized at 

 various localities along the Atlantic coast by geologists, extending from 

 New Brunswick through Maine, New Hampshire, Massachusetts, Penn- 

 sylvania, and Maryland into northern Virginia, the Carolinas, Georgia § 

 and Alabama. Some of the so-called sedimentary areas of the northern 

 Atlantic coast of the earlier geologists are now regarded as altered vol- 

 canic rocks. 



* Zur Auffassung der Chemischen Natur des Grundgebirges, Tschermak's Min. u. Petrog. Mitth. 

 1891, vol. xii, pp. 49-61. 



fG. H. Williams discusses the subject in the 15th Ann. Report, U. S. Geo]. Survey, 1895, pp. 

 663-664. 



J For a statement of the distribution of the volcanic rocks on the Atlantic coast, see Williams, 

 Jour. Geology, 1894, ii, 1-31. 



LIV— Bull. Geol. Soc. Am., Vol. 13, 1901 



