1879.] ^7 



they gradually alter to mica schist, and beyond Safe Harbor to true 

 gueiss. This lasts for about eleven miles, with its dips about evenly divided 

 by the Tocquan creek, the northern portion dipping gently N. W. and the 

 southern portion S. E. Whatever be the age of the chlorite series there- 

 fore, and whatever be the age of the Tocquan rocks, the latter interpose a 

 limit to the extent to which a change of horizon of the former may affect 

 the structure. The Tocquan anticlinal is too broad and flat and extensive, 

 not to mention its strongly marked lithological characteristics, to be any- 

 thing else than what it seems. Nobody can invert either of its limbs. It 

 is a Safe Harbor to the bewildered stratigrapher ; and a Mount Ararat to 

 the ark of the propounder of theories. This welcome element of structure, 

 however, fades out into inconclusive and rare exposures near Fishing creek, 

 after which the chlorite series begins to appear, and continues, with nu- 

 merous exposures, to a point a little less than half a mile north of Peters 

 creek, quartz entering largely into the composition of the rocks which are 

 otherwise highly convoluted, green and unctuous. Here come in the Peach 

 Bottom slates with but little time for transition, and pass, after a breadth of 

 a few hundred yards, equally abruptly, into chlorites again, and finally into 

 a greenish chloritic quartzite, which is the northern boundar}' of Peters 

 creek (when in flood). This greenish quartzite puzzled the speaker so much 

 that in his report written two years ago, but not issued, he deemed the 

 matter important enough to present two views of its age. He says, page 

 135, " The structure supposed in the section will not assign to this rock 

 contemporaneity with the Chikis quartzite, nor form a continuity with the 

 quartzites to be noted further down the river" (which are ascribed to Pots- 

 dam age). "But the interpretation of the stratigraphy here is of the 

 greatest difficulty," &c, &c. Again, page 141, another structure than that 

 adopted is given which makes "the hydromica schists in the basin of the 

 first synclinal the lower limestone slates or hydro mica schists." Not be- 

 cause of any lithological considerations, however, but solely on the hy- 

 pothesis that the column of formations appear in their normal order, which 

 needs to be established . 



In other words, if the Peach Bottom slates be established as of Hudson 

 river age, the real difficulty would seem not be a stratigraphical one ; for 

 they might be supposed to be deposited unconformably on any of the older 

 series, without the intervening members of the column. But the only diffi- 

 culty — not an insurmountable one perhaps — will be to account for the 

 alteration of the argillaceous strata characteristic of that horizon, to the 

 highly crystalline magnesia hydromicas which remind one so much of 

 what the speaker asks permission still to regard as the true chlorites — the 

 chlorites of the South Mountain. 



It is interesting in this connection, to call attention to the analysis of 

 these Peach Bottom slates, made at my request by Mr. A. S. McCreath, at 

 Harrisburg. 



The specimen is from J. Humphrey & Co.'s Quarry, half a mile east of 

 Delta, York county. 



