Notices of Memoirs — Lyman on Compass Variation. 131 



the most important, and the safest on which to rely, whenever we 

 find it. But we know that dynamic action may have more or less 

 completely effaced this structure, and in a great number of cases 

 has done so. 



We know on what lines this effacement proceeds, and with what 

 sort of new structures it replaces those it has modified or destroyed. 

 It is, however, not uncommon to find that, even in greatly affected 

 " dynamic " areas of this description, the action has not embraced 

 the wliole of the rock, and from among rolled-out, sheared, and 

 puckered schists, may come specimens showing, more or less per- 

 fectly, the contact-structures which we seeln to have good grounds 

 for always recognizing as such. , 



Compass Variation affected by Geological Structure in 

 Bucks and Montgomery Counties, Pa.^ By Benjamin Smith 

 Lyman. 



IHE Journal of the Franklin Institute, October, 1897, contains 

 an interesting paper by Mr. B. Smith Lyman, formerly State 

 Geologist to Japan, describing a remarkable coincidence between 

 the axis of a set of curves of magnetic variation in Bucks and 

 Montgomery Counties, Pennsylvania, and a great deep-seated fault 

 in the New Red strata below ending westwards in the axis of ari 

 anticlinal fold. Both the curves and the fault are shown on an 

 accompanying map. From this paper we extract the following 

 passages : — 



The magnetic curves were mapped some years before the 

 beginning of the recent Geological Survey, that for the first time 

 fully proved the peculiar structure ; but the curves had no influence 

 whatever in the interpretation of the geology, and the correspondence- 

 was not perceived until long after the geological map was printed. 



The magnetic map was made about the year 1883, by the Water 

 Department of the city of Philadelphia, for use in its excellent 

 topographical survey of the Perkiomen and neighbouring valleys 

 under Mr. Rudolph Hering. The map records the results of 

 a number of determinations of the magnetic declination made by 

 the Water Department itself and by the Coast Survey and by other 

 observers, and curves of equal declination were drawn for every 

 tenth of a degree. The curves are so extremely at variance with 

 the simple, nearly straight lines of earlier, less detailed maps, as 

 either to show extreme confidence in the accuracy of the observa- 

 tioiiS, or perhaps even to excuse a suspicion of the possible incorrect- 

 ness of the curves in some way, especially in view of the 

 acknowledged want of precision of some of the observations, and 

 the absence of any obvious topographical or other occasion for such 



^ Reprinted from the Journal of the Franklin Institute, Octoher, 1897. Mining 

 and Metallurgical Section : Inaugural Meeting, held April 28th, 1897. 



