250 Rev. 0. Fisher — Rigidity of the Earth. 



a perfect oue and one unroofed being visible. Thev are of rare 

 occurrence. The avicularian aperture has a semicircular upper lip 

 with unbroken curve when perfect, as shown by the original figure 

 and Fig. 11, but the regularity of the notches shown in Fig. 10 

 suggests a median line of weakness, perhaps indicating that the 

 species started with a notch there, the filling up of which Avas not 

 yet fully consolidated. 



In the case of E. Roivei it is clear that the outline of the avicularian 

 aperture was misrepresented in the original figure, and that it has 

 a straight lower lip and diverging sides and a convex upper lip, and 

 that the lower lip bears a long slender denticle. 



EXPLANATION OF PLATE VIII. 



(AH figures X 12 cliams.) 



Fig. 1. Semieschara labiatula. Zone (restricted) of A. quadratus, 



Upham, Hants. 

 Figs. 2, 3. S. labiatula. Trimingham. Normal specimens. 

 Fig. 4. ,, ,, Trimingham. Small form with trifoliate 



apertures. 

 Figs. 5-8. S. occlusa. Trimingham. 



Fig. 9. S. Mundesleiensis. Trimingham. Type-specimen. 



Figs. 10, 11. S. Camd. Trimingham. 

 Fig. 12. Eschara Roiuei. Trimingham. 



III. — On the Rigidity of the Earth, and on Colonel Bueeard's 



THEORr OF THE HIMALAYAS. 



By the Eev. 0. Fisher, M.A., F.G.S. 



IN the April number of this Magazine a review appeared of 

 Colonel Burrard's memoir' on the origin of the Himalaya 

 Mountains. The writer, Sir T. H. Holland, in it refers to a paper 

 of mine originally published in the Phil. Mag.,- and subsequently 

 in an amended form as Appendix jSTo. 1, 1905, Indian Survey Papers, 

 professional vol. xviii. I shall be glad to make a few remarks 

 upon the subject. After duly crediting me with having partially 

 anticipated the results now obtained by tlie Survey, by calculating 

 the deflection of the plumb-line in JSTorth India which would follow 

 from my theory of mountain compensation by a 'root' extending 

 to a depth of about 29 miles, the reviewer continues — " The variations 

 now observed are, however, more violent than those expected by 

 Mr. Fisher, for the northerly deflections of the plumb-line decrease 

 to zero at a distance of about 15 instead of over 60 miles from the 

 visible foot of the hills." I would reply that I have not calculated 

 tlie deflection at 15 miles, and it is not safe to guess a priori what it 

 would be. The numerical calculation for a given distance is tedious, 

 and I could not now undertake it.^ 



Though Sir T. H. Holland appears to admit that my theory (or 

 rather Airy's) of mountain roots goes some way to account for the 

 observed deflection, he nevertheless adds that it does not " march ", 

 like Colonel Burrard's, " with the growing belief in a solid earth." 



^ Survey of India, professional paper No. 12, Calcutta, 1912. 

 2 Phil. Mag., January 7, 1904. 



^ In the Phil. Mag., p. 24, in the formula for the attraction of the plateau, 

 there is a misprint. After the first bracket insert x. 



