obb [Die. 5, 



county, from the Peach Bottom roofing slate bell through the Gap Nickel 

 mine, aorth-eastward into the Welsh mountains. But, as the roofing slate 

 bell is several miles long, I can see no important connection between the 

 trap at one end of it and its inetamorphisra. 



Prof. Frazer feels sure that the roofing slates arc part and parcel of the 

 chlorite slate formation which makes such a show along the river for miles 

 north of the quarries, But the structure is very obscure. To the north of 

 the (south dipping) chlorites, a hold, double-crested anticlinal (of Tocquan 

 creek) crosses Lancaster and York county, and is finely exposed upon the 

 two banks of the Susquehanna river, bringing up massive gneisses, &c, evi- 

 dently referable to our Philadelphia gneisses, to those of the Welsh moun- 

 tains west of the Schuylkill river, and to those of the Highlands of New 

 Jersey and New York states. The chlorite slates are always seen in this 

 region in juxtaposition with limestones which we feel confident are No. II 

 ("Magnesian" "Calciferous") ; hut the structural connection is not yet 

 quite satisfactory. Mr. 0. E. Hall is disposed to look upon them all along 

 the Chester county "south valley hill," and across the Schuylkill into 

 Philadelphia, and towards Trenton, as No. Ill (Hudson river) metamor- 

 phosed. 



Everything points towards nonconformable basins or outlying patches of 

 metamorphosed Silurians in the heart of our Azoic country of southern 

 Pennsylvania and Maryland, and this discovery of B. flezuosa leaves very 

 narrow room for further doubt on the subject. 



Prof. Frazcr gave a partly detailed description of the sec- 

 tion along the Susquehanna, — an analysis of the difficulties 

 he encountered in making;; out its true structure, — and the 



O 7 



doubts which still hang over the relationship of the roofing 

 slate belt to the chloritic, mica slate and gneissoid areas, on 

 each side of the great Tocquan anticlinal. 



Prof. Frazer said that in reference to the effect of the 

 determination of the Peach Bottom slates as of Hudson 

 river age, a word of explanation would make its extent 

 clear. 



The Susquehanna section was prepared carefully foot by foot with a per- 

 fectly accurate 200 ft. — 1 inch R. R. plotting in the hand. The exact 

 position of every station (the stations were all 100 feet apart) and the 

 outline of the shore and curves in the road were given on the plot, 

 while the inner side of the outer rail was painted with the number corre- 

 sponding to each station. Locations were therefore almost perfectly ac- 

 curate. From Columbia to Turkey hill (± 5 miles south) was filled with 

 limestone. Chlorite slates come in abruptly at Turkey hill, and last along 

 the shore (still going smith) to within a mile or so of Safe Harbor, when 



