128 PROCEEDINGS OF THE ACADEMY OP 



The celestine is of the fibrous variety, the fibres being perpen- 

 dicular to planes of bedding. It is rather pale, the greater part 

 of it having a very faint blue tint. 



April 27. 



The President, Dr. Ruschenberger, in the chair. 



Twenty-four members present. 



The By-Laws of the Academj^ -were amended, as follows: 

 Art. 2, Chap. 2, in place of " Minors shall not be eligible as mem- 

 bers or correspondents," the following words are substituted : 

 " Pei'sons not under sixteen years of age may be elected members, 

 provided their nominations be first approved by the Council. 

 Members under the legal age of twenty-one are not entitled to 

 vote at any meeting of the Academy, nor to serve on committees." 



On the Structure of the York County Valley Limestone , and on 

 Micro-photography of Minerals. Prof. Frazer asked the per- 

 mission of the Academy to put on record two observations made 

 since the last meeting. The first concerned the structure of the 

 York County limestone valley. H. D. Rogers (whose work was 

 so accurate and full of thought that corrections can only be 

 hazarded after careful consideration), in his report of a section 

 down the Susquehanna from Wrightsville to Havre de Grace, 

 speaks of two folded anticlinals which separate the main synclinal 

 basin of amoral limestone from the smaller one, which crosses the 

 river from Lancaster County near Cabin Branch Run. 



The dips as observed and recorded by my party last season 

 fully justify this interpretation, and were it not for other facts 

 not known at the time the above section was made, no one could 

 hesitate to accept this explanation. 



But on comparing seven sections made across the lower Silurian 

 measures from Littlestown to Wrightsville, it was found that in 

 every case but the latter, there seemed to be abundant evidence of a 

 vault along which the southeastern half of the valley had been torn 

 away by an upthrow, and the remaining limestone abutted on the 

 lower side of the older slates. It was found that the supposed 

 double anticlinal wave structure depended on a single dip, which 

 it only needs to suppose were local in character to bring this sec- 

 tion into conformity with the rest. The symmetrical character 

 of the valley also was based upon a single dip in this latter case, 

 not at all inconsistent with a view which would harmonize all the 

 sections. 



The other point which Prof. Frazer desired to record was in 



