120 PROCEEDINGS OF THE ACADEMY OF [1890. 



bedded in the gneiss in large layers one to six meters in thickness 

 and that four modes of alteration of this dunyte occur, viz. : — 



1, Chalcedonic. "When all the bases have disappeared and the 

 chalcedony remains as an exceedingly cellular mass of thin scales 

 and plates parallel or anastomosing with the greatest irregularity, 

 a chalcedonic schist or siliceous sinter is the result, often bearing 

 some resemblance to a buhrstone." 



2. Hornblendic. " The final result is a green actinolite rock or 

 schist or grayish-white amphibolyte or tremolyte schist. 



'x Talcose. 



4. Ophiolitic. 



5. Dioritic." 



One prominent feature of the most of the Pennsylvania outcrops 

 above described is the abruptness of their appearance and disappear- 

 ance, in this respect resembling far more plutonic masses than inter- 

 stratified beds or synclinal basins. If interstratified they should be 

 somewhat lenticular in outline. If synclinal the adjacent dips being 

 steep as seems to be always the fact they should also show a more or 

 less lenticular or boat-shaped outline. On the contrary, where ex- 

 posed they often appear abruptly and end abruptly, well shown at 

 the outcrops east and northwest of Radnor station, on the Radnor 

 and Chester road near Darby creek, in Marple, at Blue Hill where 

 a hill or ridge of considerable height is serpentine to the southwest 

 bounded by Laurentian on the northwest and wholly Laurentian on 

 the northeast, the strike of the Laurentian being nearly south, that 

 of the hill nearly northeast and southwest.^ Nevertheless the posi- 

 tion of the serpentine belts close to the Laurentian on the one side 

 and with mica and hornblende schists on the other, seems to for- 

 bid our regarding them as intrusive rocks. The subject is beset 

 with difficulties but the theory of interbedding, notwithstanding the 

 difficulty of explaining the want of continuity and the abrupt ap- 

 pearance and disappearance, seems the only one tenable. It is true 

 that the great Conshohocken trap dyke strikes almost with the adja- 

 cent rocks, but nevertheless it does not do so exactly, its bearing west- 

 wardly being more southerly, so that at the Schuylkill it is in the 

 hydromica schist of the South (Chester) Valley Hill. It then cuts at 

 a very acute angle the limestone, Potsdam and, mica schists of Cream 



1 The outcrops of the Radnor-West Chester belt are on a hne nearly east and 

 west but the outcrops themselves all cross this line bearing more N. E. and S. W. 

 except tliat northeast of Radnor station. 



