McGee — Formations of the Middle Atlantic Slope. 367 



of aqueous vapor on atmospheric refraction, varying baro- 

 metric height, the minute variations in the grating space, 

 failure of thermometer to give temperature of grating exactly, 

 and countless others which will suggest themselves only too 

 readily. 



Aside from the use of gratings, decidedly the most hopeful 

 method as yet suggested is that due to Michelson and Morley.* 

 Theoretically the plan is particularly simple and beautiful, con- 

 sisting merely in counting off a definite number of interference 

 fringes by moving one of the interfering mirrors and measur- 

 ing, or laying off upon a bar, the resulting distance. The 

 mechanical difficulties in the way, are however formidable, 

 and whether or no they can be surmounted only persistent 

 trial can show. The possible sources of error are of much the 

 same type and magnitude as those involved in the comparison 

 of standards of length, and if these errors are avoided, the 

 uncertainty concerning the standards still remains. Whether 

 or no the practical errors of the method are greater or less 

 than with gratings only experience can prove. Certainly if 

 the method is capable of giving exact results it is in the hands 

 of one able to obtain them from it. 



In closing this paper I can only express my sincerest grati- 

 tude to the various friends who have done all in their power 

 to facilitate my work, and especially to Professor W. A. 

 Rogers who has been tireless in his endeavors to determine the 

 true value of the standards of length ; to Mr. J. S. Ames, Fel- 

 low in this University, who has given me invaluable aid in the 

 work with metal gratings ; and to Professor Rowland who has 

 furnished all possible facilities and under whose guidance the 

 entire work has been carried out. 



Physical Laboratory, Johns Hopkins University, March, 1888. 



Art. XXXI. — Three Formations of the Middle Atlantic 

 Slope; by W. J. McGee. (With Plates YI and VII.) 



(Continued from page 330.) 



The Columbia Formation. 



General Characters. — The Columbia formation exhibits two 

 phases which, although distinct where typically developed, inter- 

 graduate. The thicker and more conspicuous phase occurs com- 

 monly along the great rivers at and for some miles below the 

 fall line, and may be designated the fluvial phase ; while the 

 thinner generally forms the surface over the remainder of the 

 Coastal plain, and may be designated the interfluvial phase. 



* This Journal, III, xxxiv, 427. 



