1885.] NEW-YORK MICROSCOPICAL SOCIETY. 143 



Scotch granytes imported into this city as building stones), 

 during the folding up of the strata into mountains. The heat 

 produced in the course of this tremendous compression of the 

 rocks has left its record in the resulting crystalline condition of 

 these rocks and in the fluid-cavities which they contain. 



An investigation of great interest was prosecuted by Messrs. 

 H. C. Sorby'^ and J. C. Ward,'* of England, to ascertain the con- 

 ditions of temperature and pressure which have prevailed during 

 the formation of certain Scotch and Cornish rocks, especially 

 granytes and elvans, in which such partly filled cavities occur. 

 It was established by them that the bubble floating upon the 

 liquid represented the vapnr-filled vacuity left by the contrac- 

 tion, by cooling, of the liquid which must have originally filled 

 the heated cavity. The determination of the temperature (89° 

 -356° C.) at which the liquid could be artificially compelled to 

 resume its original condition and volume at the time of the 

 genesis of the crystalline rock, and of the relative change in its 

 volume, gave a measure of the depth below the surface at which 

 this temperature prevails, and of the superincumbent pressure 

 necessary to produce the required tension in the cavities. By 

 reasonings of this kind, it was estimated that the Scotch granytes 

 were consolidated under pressures varying from sixty-nine 

 thousand to seventy-eight thousand feet of rock at a temperature 

 between 200° and 360° C. (a dull red). These cavities may be 

 looked upon as offering a like record of past thermal experi- 

 ences to that which we now obtain by means of maximum ther- 

 mometers and pyrometers. For such investigations it was 

 necessary to select peculiarly symmetrical long cavities of tubu- 

 lar or cylindrical form, the volume of which could be measured 

 approximately, and that of the liquid they contained. Such 

 tubular cavities are particularly common along certain planes in 

 the white Brazilian topaz, and are often connected irregularly in 

 pairs or even in groups of parallel tubes. In one case, a pair so 

 connected, in a U-shaped figure, contained bubbles and portions 

 of liquid (carbon dioxide) so delicately balanced in the opposite 

 arms, that the whole arrangement could be made artificially to 

 act very much like a differential thermometer. 



We owe to Geo. W. Hawes the detection in one instance, in 



"Quar. Jour. Geol. Soc, 1858, XIV., pp. 453-500 ; and Min. Mag., 1877, I., p. 41. 

 "Quar. Jour. Geol. Soc, 1875, XXXI., p. 568 ; and 1876, XXXU., p. 1. 



