70 R. D. OCdham — Essays in Theoretical Geology. 



The above form is adapted for use with Klein's solution (Boro- 

 tungstate of Cadmium), which has been found the best for practical 

 use for densities below 3-28, as it has not the highly poisonous 

 character of the Sonstadt solution (potassium iodide and mercuric 

 iodide) and does not decompose and discolour so easily as methylene 

 iodide. Above 3'28, methylene iodide, with or without the addition 

 of iodoform (as proposed by Eetgers),' must be used. Here, as the 

 use of india-rubber is inadmissible, the pinchcock must be replaced 

 by a glass tap (but the bore need not be the same as that of the glass 

 tube, as is usually necessary in apparatus of this nature) ; and 

 instead of using the india-rubber Ci, the end of the thistle-tube C 

 and the base of the conical part of the funnel must be ground so 

 as to fit one another, and the rod Z) must closely fit the tube. , 



The tube C has some external similarity to the plug used in the 

 useful apparatus devised by Mr. Smeeth ; "^ but there the plug is not 

 perforated, and there is no resemblance in the mode of action of the 

 two forms. 



I ought to add that all my work with heavy liquids has been 

 carried on at the Eoyal College of Science, where Professor Judd 

 has kindly given me every facility in my endeavours to find the best 

 practical form of instrument neither too expensive nor too delicate 

 for students' work. 



IV. — Essays in Theoretical Geology. 

 By E. D. Oldham, A.R.S.M., F.G.S., 



of the Geological Survey of India. 



The Age and Origin of the Himalayas, with Especial Eefer- 

 ENCE TO the Eev. 0. Fisher's Theory of Mountain Formation. 



Tlie Facts [continued from p. 18). 



IHAYE already shown that the northern boundary of the Indo- 

 Gangetic alluvium is a structural one, and that the rock area 

 immediately to the north of it has been elevated, while the nature of 

 the boundary on the south, the deep imbayments of the alluvium, 

 the gentle manner in which the older rocks slip under it, and the 

 inliers, alike show that, on the south, the alluvium has gradually 

 encroached on the rock area by the subsidence of the latter. 



I have also shown that at one period the present demarcation 

 of the Peninsular and Extrapeninsular areas did not exist, and that 

 the latter extended across the area now occupied by the Gangetic 

 alluvium into what is now Sikkim. From this we may conclude 

 that during the rise of the Himalayas the formation of the depression 

 now occupied by the alluvial deposits of the Indus and Ganges 

 has proceeded, and that, just as its northern boundary has been 

 encroached upon by the gradual southward extension of the Himalayan 

 region, so it has itself encroached on the rock area to the south, 

 and necessarily at a greater rate. 



1 J. W. Eetgers, Neues Jahrbuch, 1889 (2), p. 185. 



- Scientific Proceedings of the Eoyal Dublin Society, vol, vi. (new series 1888), 

 p. 58. 



