6 Proceedings of the Boyal Physical Society. 



presence of bituminous matter. If a section of a crystal of 

 brown quartz be placed under the microscope, many fluid 

 cavities may be observed ; but if we expose it to a tempera- 

 ture equivalent to that of iron at a dull red heat, we find 

 that not only have these cavities been emptied of their fluid 

 contents, but the colour has also been discharged. Now 

 this very curious fact has been long taken advantage of by 

 the jewellers in decolorizing, specimens of brown quartz 

 from Cairngorm, which were too dark for their purpose. 

 How then can we suppose this brown quartz to have re- 

 tained its bitumen under such a fiery ordeal as we are led 

 to believe the granite which envelops it has undergone ? 

 When we ask the question, we are very coolly informed by 

 the advocates of Plutonism, that the granite was then under 

 immense pressure, by means of which not only the fluid 

 contents of the cavities, but the bitumen was retained. 

 Had these gentlemen given any attention to mineralogy, 

 they would soon be convinced that in this instance at least, 

 the argument was vain, as in all cases where the crystals of 

 brown quartz are found they occupy large cavities in the 

 granite, a condition not likely to obtain under such immense 

 pressure as they assume. Another observation confirmatory 

 of these views has been made by a young chemist, Mr Scott 

 of Dublin, who has shown that felspars, in many of the 

 Irish granites, are decomposed at low temperatures, and 

 could not have existed in their present chemical conditions 

 under great heat. The most sincere Plutonists are now 

 wavering in their opinion regarding the high temperatures 

 to which the primary rocks were exposed ; even the ex- 

 periment of Hall, of converting chips of limestone into 

 marble under great heat and pressure, is now, if not entirely 

 ignored, greatly doubted, and the Museum of the Koyal 

 Society cannot clear up the mystery, as no specimens of 

 Hall's have been conserved. Even the first discovery of 

 Hutton, of the intruding granite at Glen Tilt, has been 

 pushed aside as useless by the present writers on the igneous 

 origin of . the primary rocks. This opinion is strongly ex- 

 pressed by the Eev. Vernon Harcourt, who, in a report made 

 to the British Association in 1860, on the effects of long con- 



