132 Notices of Memoirs — Lyman on Compass Variation. 



great irregularity. But the curves are in the main beautifully 

 confirmed and thoroughly vindicated by the underground geology. 



The striking feature and dominant peculiarity of the curves is 

 a very strong bend convexly north-eastward near New Hope and 

 Lambertville, on the Delaware ; but gradually changing towards 

 the west, so that the curves near Shwenksville and Boyertown point 

 still more sharply south-eastward. The axis of the bend in the 

 curves is, then, itself greatly bent, neai'ly to a right angle. The 

 Geological Survey of the two counties, begun at the end of 1887, 

 has proved beyond a question the existence of an enormous fault, 

 of about 14,000 feet, in the rock beds, almost precisely on the 

 line of the Delaware Eiver end of that magnetic axis, and 

 following the same course past Doylestown, gradually dying out, 

 and west of that town turning north-westward, passing north of 

 Shwenksville, disappearing there as a fault, but continuing as 

 a sharp anticlinal to the border of the New Red and of Montgomery 

 County, 5 miles north-east of Boyertown. 



The geological structure of the map of 1893, published by the 

 State Geological Survey, was drawn without the least reference to 

 the magnetic curves, and, indeed, without any knowledge at that 

 time of the slightest correspondence between them and the geology. 

 The geological map gives the direction and amount of the dip at 

 a couple of thousand points, amounting to a complete demonstration 

 of the structure, and to a full proof of the situation and extent 

 of the fault and of the sharp anticlinal into which the fault runs. 

 The topography also given on the same map shows that there 

 is no one strongly-marked ridge following the course of the axis 

 of the magnetic curves. Indeed, there are more decided topo- 

 graphical indications in the way of long, rather high ridges in other 

 directions. Furthermore, the form of the outcropping rock beds, 

 sedimentary or igneous, does not correspond in any degree with the 

 magnetic curves. 



Moreover, some light is perhaps thrown upon the obscure subject 

 of terrestrial magnetism. It is true, the nature of the relation 

 between the magnetic and geological phenomena is not so easily 

 determined ; but it seems to become certain that the internal 

 structure of the earth's crust has an important influence upon 

 terrestrial magnetism, even if it be not in any degree its first cause. 

 Terrestrial magnetism and its changes have sometimes been con- 

 sidered explainable by solar influences alone, no longer by direct 

 action of the sun as a magnet, but by the sun's heating the 

 atmosphere or the earth's crust. The present phenomena seem, 

 however, to point to more strictly terrestrial processes as the true 

 cause, and to suggest that the solar influence may partly at least 

 be exerted through the attraction of gravitation as well as through 

 heat. The enormous and locally unequal strains produced by the 

 contraction of the earth's crust in cooling would be particularly 

 liable to be affected by the presence of a deep fault or by a sharp 

 anticlinal. Such lines would be places where the crust has yielded 

 and is readier to yield, and consequently where the strain has 



