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dynamic geology, were left almost unregarded by the great masters 
of the science, who generally accepted the speculations of Hutton 
and the experiments of Hall, as demonstrating the igneous origin of 
the primary rocks. 
The author stated that the Huttonian theory was most ably 
attacked, and, in his opinion, overthrown by Dr Murray in his 
“ Comparative View of the Huttonian and Neptunian Systems of 
Geology,” a work most unaccountably overlooked. Since that time 
it had suggested itself to the sagacious mind of Davy, that the 
occurrence of fluids in the cavities of crystals seemed to point to an 
aqueous origin. He also alluded to the writings of Brewster, Sive- 
wright, and Nicol, in the same field ; also to Becquerel, Fuchs, 
Bischoff, and Delesse, who have taken up the subject of the aqueous 
origin of rocks from a chemical point of view. The author then 
laid before the Society the result of ten years’ experimental investi- 
gation into the structure of rocks relative to their formation, more 
particularly granite. While examining microscopically the various 
pitchstone veins abounding in Arran, he was much struck with the 
similarity of their structure, and the marked difference they ex- 
hibited when compared with sections of granite and its various 
mineral constituents. On extending his observations to obsidian, 
marekanite (a volcanic glass from Lake Marekan in Kamtschatka), 
and also to the well-known glassy obsidian of Bohemia, he found 
they all exhibited a structure analagous to the pitchstones of Arran. 
He further found that sections of glass slags, where the heat had 
been long continued, combined with slow cooling, all presented the 
same appearances as the sections of pitchstone. 
This structure, peculiar to igneously formed substances, he found 
usually to radiate in a stellate form ; and though many slags 
showed large stars visible to the naked eye, the stellate structure is 
more easily observed by the aid of the microscope. The character 
is so marked that no one whose eye is tutored to microscopic observa- 
tion can fail to recognise at once a mineral substance of igneous origin. 
In granite, on the other hand, the structure, as seen by the mi- 
croscope, is as persistent as in pitchstone, glass, and obsidian, but 
totally different. 
In the many experiments which the author had tried with gra- 
nites from various localities, he had never succeeded in obtaining 
one instance of stellate structure, while the constant occurrence of 
