8 1 4 THE JO URNAL OF GEOL OGY. 



solidation and subsequent alterations. (2) In the northern part 

 of the range a brilliantly colored acid volcanic rock predomi- 

 nates. 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 dif- 

 ferent 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 strticture. — -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? — 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 min- 

 erals 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. 



