110 PROCEEDINGS OF THE AMHERST MEETING 



STRATIGRAPHY OF THE CRYSTALLINE SCHISTS OF PENNSYLVANIA AND 



MARYLAND 



BY ELEANOR BLISS KNOPF AND ANNA I. JONAS 



(Abstract) 



This paper presents the results of field studies in the crystalline schists of 

 southeastern Pennsylvania and Maryland. The crystalline schists of Balti- 

 more County, Maryland, are Precambrian. The base of the section is the 

 Baltimore gneiss, a highly crystallohlastic paragneiss injected in some places 

 by granitic magma. Overlying the Baltimore gneiss is the Glenarm series of 

 recrystallized sediments, whose thickness is provisionally estimated at 8,000 

 to 10,000 feet. These rocks show a metamorphism whose intensity decreases 

 progressively from highly crystalline schists at the base to mildly anamor- 

 phosed sediments at the top. The lower formations of the Glenarm series are 

 cut by plutonic intrusions, and the upper formations contain amphibolite 

 schists that are probably metamorphosed volcanics and may be the equivalent 

 of the Precambrian Catoctin lavas. 



The Precambrian core of the Reading-Durham Hills in Pennsylvania is a 

 completely recrystallized paragneiss injected by various plutonic rocks. The 

 cover of Paleozoic sediments is very slightly altered. In Mine Ridge Hill, 

 which is the western termination of the upland that lies on the northwest side 

 of Chester Valley, the Precambrian core is lithologically similar to the rocks 

 of the Reading Hills. The sedimentary series on the flanks of Mine Ridge 

 Hill is thoroughly crystalline, but is considered to be Paleozoic and to repre- 

 sent the metamorphosed equivalent of the Lower Cambrian arenaceous sedi- 

 ments in the Hellam-Chickies ridge, northwest of Mine Ridge Hill. 



The intensity of metamorphism in the region does not die out gradually 

 from southeast to northwest across the strike of the folds, but there are three 

 belts of intense metamorphism that lie parallel to the folds in northern Mary- 

 land and southern Pennsylvania, and these belts are separated by areas of 

 diminished rnetamorphic intensity. There is also a waning of metamorphic 

 intensity from the lowest to the uppermost Precambrian formation, succeeded 

 by a renewed intensity in the Paleozoic rocks of Mine Ridge Hill. 



Therefore the metamorphism can not be explained as a result of simple 

 dynamic action where the intensity is conditioned by distance from the locus 

 of initial yielding to the folding force, nor as simple load metamorphism con- 

 ditioned by depth of burial. A combination of load inetaniorphism and re- 

 gional contact metamorphism is a possible explanation of the localization of 

 areas of intense metamorphism. Further stratigraphic work will be necessary, 

 however, before any theory of metamorphism for the region can be proved. 



Read from manuscript by the senior author. 

 Discussed by Florence Bascom and G. H. Ashley. 



Discussion 



Professor Bascom stated that she could not let the paper pass without com- 

 menting on what must be evident to the Society, the value of the kind of de- 

 tailed and elaborate study which the paper represented. The Piedmont lias 



