H. J. Biddle — Surface Geology of Southern Oregon. 475 



But the difficulties of the elastic theory do not end with the 

 law of double refraction, although they are there more con- 

 spicuous on account of the definite and simple law by which 

 they can be judged. It does not easily appear how the equa- 

 tion of energies can be made to give anything like the proper 

 law of the dispersion of colors. Since for given directions of 

 the wave-normal and displacement, or in an isotropic body, 

 B ND is constant, and also A D and o m except so far as the type 

 of the vibration varies, the formula requires that the square of 

 the index of refraction (which is inversely as Y 2 ) should be equal 

 to a constant diminished by a term proportional to the square 

 of' the period, except so far as this law is modified by a varia- 

 tion of the type of vibration. But experiment shows nothing 

 like this law. Now, the variation in the type of vibration is 

 sometimes very important, — it plays the leading role in the 

 phenomena of selection absorption and abnormal dispersion, — 

 but this is certainly not always the case. It seems hardly 

 possible to suppose that the type of vibration is always so vari- 

 able as entirely to mask the law which is indicated by the 

 formula when A D and 5 D (with B ND ) are regarded as constant. 

 This is especially evident when 'we consider that the effect on 

 the wave-velocity of a small variation in the type of vibration 

 will be a small quantity of the second order.* 



The phenomena of dispersion, therefore, corroborate the con- 

 clusion which seemed to follow inevitably from the law of 

 double refraction alone. 



Art. XLI. — Notes on the Surface Geology of Southern 

 Oregon ; by Henry J. Biddle. 



During the Summer of 1887, the writer had occasion to 

 visit that portion of southern Oregon which lies within the 

 area of interior drainage, and forms the northwestern part of 

 the Great Basin. In the intervals of other work, some notes 

 on the surface geology of the region were made, which, though 

 necessarily fragmentary and incomplete, may yet be of sufii- 



which express the direct action of the ponderable molecules on the ether. So far 

 as the (quite limited) reading and recollection of the present writer extend, those 

 who have sought to derive the law of double refraction from the theory of elastic 

 solids have generally either neglected this direct action — a neglect to which 

 Professor Stokes calls attention more than once in his celebrated " Report on 

 Double Refraction" (Brit. Assoc, 1862, pp. 264, 268,) — or taking account of this 

 action they have made shipwreck upon a law different from Fresnel's and con- 

 tradicted by experiment. 



* See volume xxiii of this Journal, pp. 271, 272, or Lord Rayleigh's "Theory 

 of Sound," vol. i, p. 84. 



Am. Jour. Sci— Third Series.— Vol. XXXV, No. 210.— June, 1888. 

 29 



