27 



100 grammes of water require 35 '5 grammes of salt in order to saturate the 

 solution. If the law of REGNAULT holds good at higher temperatures, then 

 a temperature of 307 C. would be required to enable the liquid in the above- 

 mentioned cavity to dissolve the salt crystal. 



Unfortunately, the assumptions involved in this method, as in that of 

 SORBY, are so doubtful that little confidence can be placed in the numerical 

 result. That REGXAULT'S law should hold good with sufficient accuracy to 

 yield even approximate results so far above the point to which it has been 

 experimentally verified, and under such enormous pressures as exist in the 

 earth's crust pressures, be it remembered, which may be vastly in excess of 

 that due to the tension of water-vapour at the calculated temperature (87 

 atmospheres) is extremely doubtful. Another objection to all conclusions 

 of the kind above referred to lies in the fact that fluid inclusions are some- 

 times of secondary origin ; and we have, at present, no safe criterion by which 

 we can judge in all cases as to the original or secondary character of any 

 particular inclusion. 



VOGELSANG (1) appears to have been the first to doubt the original 

 character of the fluid inclusions of the minerals of eruptive rocks. 

 He calls attention to the fact that they usually lie in planes, which 

 may be regarded as cracks, and he considers them to be "cavities 

 which, in most cases, have not been quite filled up with liquid by 

 secondary injection." This conclusion is confirmed, so far as a particular 

 rock is concerned, by A. A. JuLlEX/ 2 ' Fluid inclusions of the normal kind 

 abound in the quartz of the fibrolitic gneiss of New Rochelle. When 

 examined with a high power (i-iii. objective) the rock is seen to be traversed 

 by very numerous and exceedingly minute fissures, partly in planes which are 

 approximately parallel, at least, within the area of the thin section, and 

 partly as branching cracks in an irregular net-work." Needles of fibrolite 

 are scattered throughout the quartz grains in this rock, and where the fissures 

 cross the needles they are represented by minute dark lines. The fluid 

 inclusions are, for the most part, limited to the quartz grains and the planes 

 in which they lie correspond exactly with the cracks in the fibrolite needles. 

 The author's general conclusions are as follow: "All these phenomena are 

 interesting evidences of the microscopic results of the internal and gradual 

 movements within the mother rock, in the process of folding. The mass has 

 been repeatedly seamed by minute fissures, yielding the plasticity long 

 recognised in rock-niasses of apparently the greatest rigidity, and repeatedly 

 re-cemented by siliceous films, deposited out of the concentrated and heated 

 solutions which saturated the rock." 



KALKOWSKY (3) has also called attention to the correspondence between the 

 lilies of fluid inclusions and cracks in the fibrolite needles in a rock from the 

 Eulengebirge, similar to the one on which JULIEN'S observations were based. 



Dr. HICKS (4) refers to the probable secondary origin of inclusions in the 



(1) Philosophic der Geologie. Bonn, 1867, p. 155. 



(2) On the fissure-inclusions in the fibrolitic gneiss of New Rochelle. Amer. Jour. 

 Micro. Sci., 1879. 



(3) Die Gneissl'oruiation des Euleugebirge. Leipzig, 1878, p. 7. 



(4) On Cambrian Conglomerates in Anglesea and Caernarvon. Q.J.G.S., vol. XL., p. 11)4. 



