282 [September. 



of primary or crystalline limestone mixed up with seams of quartzite 

 and miea slate. But when carefully examined it turns out to be an 

 exposure of the summit of a sharp anticlinal axis, of that order which 

 is so common in much-disturbed regions of all ages, and especially 

 worthy of study in the narrow basins of the coal. A diagram (o) 

 will show this structure better than any de- 

 scription. The masses of quartzite and mica 

 slate in the walls of the quarry are inverted 

 tongues of the roof-rock, and the apparent 

 horsebacks in the floor are the points of cor- 

 responding tongues of the floor-rock. 

 The occurrence of such cases of fork-tongued anticlinal structure 

 in so obscure a region as that of the Trenton-Philadelphia-Baltimore 

 primary rocks, is important in the present state of our knowledge of 

 this difficult region. No good field analysis of its structure has yet 

 been made. The Final Report of the Geological Survey of the State 

 gives no satisfiictory account of it, enters into no details, and acknow- 

 ledges that the colored limits of the formations upon the map were 

 traced in great measure by conjecture. The mere appearance or 

 outcrop of a primary limestone bed anywhere in this region would be 

 a happy accident, and should be used as a clue to the structure in the 

 way Sir Williaui Logan has used the outcrops of limestone in Canada 

 for working out the structure of the Laurentian formations. 



But the rke and fall in anticlinal of a. limestone bed among these 

 altered sandstones and magnesian slates is much more important and 

 suggestive, both in reference to the general interpretation of the dips 

 of the region and in reference to the sub- or super-position of the 

 Chester County limestone, wlwch to appearance passes underneath 

 the southern hills in the direction of this fork-tongued anticlinal of 

 limestone, — but which most of the Pennsylvania geologists have re- 

 garded as a Lower Silurian synclinal with its southern slope over- 

 thrown so as to make a south dip where a north dip ought to be. 



In itself the shape of this anticlinal is curious enough and strictly 

 analogous not only to those in the coal, a specimen of which is given 

 in figure (h), but also to innumerable cases in 

 the gneiss around Philadelphia. Just after en- 

 tering Fairmount Park and ascending the river 

 road to what was once Pratt's mansion, the ho- 

 rizontal streaks of (juartz in the gneiss are seeu 

 to form a series of closed zigzags of the same 

 kind. A far more extensive and extraordinary system of these plait- 



