6 Proceedings of the Royal 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 Eoyal 
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. Yernon Harcourt, who, in a report made 
to the British Association in 1860, on the effects of long con- 
