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THE JOURNAL OF GEOLOGY.

solidation and subsequent alterations. (2) In the northern part of the range a brilliantly colored acid volcanic rock predominates. It is porphyritic or non-porphyritic, amygdaloidal or compact. It is accompanied by pyroclastics and breccias. It is sometimes sheared into a fissile slate or sericite schist. (3) Toward the south and extending into Maryland a dark green basic volcanic rock predominates. This is also amygdaloidal or compact, accompanied by pyroclastics or breccias, and usually rendered schistose by pressure.

The acid volcanics.—While some of the acid volcanics are typical quartz-porphyries, others possess a groundmass which, although holocrystalline, contain the evidence of a distinctly different original character. It it this important portion of the acid flow, which will be more particularly treated in what follows. Certain conspicuous structures of the groundmass contain the history of the rock and merit a detailed description.

Fluidal structure.—The fluidal structure, which is a familiar one to all students of rhyolitic lavas, is a marked feature of these pre-Cambrian volcanics. Delicate lines of flow are brought out in great detail by weathering or are painted in brilliant colors in the material washed by the mountain brooks. The microscope shows globulites of magnetite, and hematite, and indefinite opaque microlites following sinuous lines of flow, twisting around the phenocrysts and imparting to them the appearance of eyes.

Micropoikilitic structure.[1]—This name has been given to a structure which is almost universally present in the acid and more rarely in the basic volcanics of the South Mountain. It consists in the presence in the groundmass of irregular quartz areas enclosing micolites of lath-shaped feldspars or other minerals with independent optical orientation. This structure between crossed nicols gives a pronounced mottled or patchy appearance to the groundmass, an appearance which has not infrequently been noted in volcanics of all ages. It has been variously described, usually without being named, in quartz-

  1. ↑ G. H. Williams: On the Use of the Terms Poikilitic and Micropoikilitic in Petrography.Jour. of Geol., Vol. I., No. 2, February-March, 1893, pp. 176-179.
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