1890.] NATURAL SCIENCES OF PHILADELPHIA. 87 



Mr. Hall ^ thinks that what has been taken for the dip of the 

 gneisses and schists in southern and southeastern Delaware County is 

 cleavage, and that instead of their lying at high angles, they are not 

 far from horizontal, overlying each other, the Philadelphia gneisses 

 the lowest, the Manayunk schists the next, and the Chestnut Hill 

 schists the highest. He places the serpentines, except those of Flush- 

 ing and of Cresheim creek, all in one horizon and above the Chestnut 

 Hill schists. 



The hard hornblendic gneiss, as heretofore stated, he deems Lau- 

 rentian underlying unconforniably the other rocks and exposed by 

 erosion along the streams. So far as my examination has gone, 

 and I have examined many hundreds of outcrops, I have been 

 unable to verify ]\Ir. Hall's conclusions, and I believe the dips to be 

 almost without exception at high angles generally towards the north- 

 west, except along Chester Creek northward of Lenni, where the 

 prevailing dip is southeast and less steep. Rejecting the segregated 

 granitic veins often found, the distribution of the rock-making ma- 

 terials will be found not at low angles, but nearly vertical. Very 

 evident is this when one of the many hornblendic beds occurs be- 

 tween micaceous and feldspathic rocks, but careful search has shown 

 it in nearly every locality where the exposure was at all good. The 

 want of uniformity in the various strata makes this all the more dis- 

 tinct. There is one exposure in Deshongs' quarry on Ridley creek, 

 west of the Philadelphia and West Chester road at which mica 

 schists appear to rest unconformably upon the Fairmount gneiss, 

 but the exposure is far from good, and there is no other to my knowl- 

 edge. 



If ]\Ir. Hall's theory is correct, we should find in ascending the 

 hills from the creeks a decided change in the rocks ; but, except 

 that due to decomposition, no such change exists ; on the summits, 

 where exposed l)y road cuttings, the decomposed rocks are identical 

 with those in the adjacent valleys. Besides this, the harder gneisses, 

 both feldspathic and micaceous and hornblendic, which occur 

 beyond all doubt interstratified in the mica schists, cannot be dis- 

 tinguished lithologically, from those of the areas about Chester creek 

 colored Laurentian on the map in C". The peculiar weathering of 

 the Laurentian is absent, there is no blue quartz, and there is no 

 line of demarkation between the areas colored Laurentian, and those 

 colored schists on Mr. Hall's map, beyond Avhat would be easily and 



^ Second Geolog. Survey Pa., Vol. C= pp. 2 and 59, etc. 



