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bringing optical science to bear upon them, through such 
instruments as the stauroscope, polariscope, &c. The authors 
of this new and more excellent way were two Englishmen, 
Messrs. Nicnors and Sorsy. Zrrxen took up the new study in 
1862, at Vienna, and Vocrxsane at Delft; but to RosENBUSCH, 
of Friburg University, is due the credit of first testing the 
mineral constituents of rocks by the use of polarised light. 
He then gave an account of Durocusr’s theory of the two 
magmas of the fluid melted rocks, which is in the main 
undoubtedly correct, though little trace of the original magmas 
can be detected now. The magmas or layers are known as the 
Acidic and Basic series; the former containing on the average 
70 per cent. and upwards of silica or silicic acid, and the latter, 
not so rich in silica, containing on an average not more than 
50 per cent. or less. The two series are otherwise known to 
Petrologists as Orthoclastic and Plagioclastic; or again as the 
Felspathic and the Augitic series. The difference between 
crystalline and amorphous matter was described, and the 
embryonic forms on incipient crystals, called “ crystallites,” 
were passed under review. The terms assigned to the two 
main groups of Igneous rocks, viz., the Vitreous and Crystalline, 
were enumerated,—these terms, it must be noticed, were only 
names of typical groups. Following the arrangement given by 
Roriey, the “Vitreous” or “ Glassy,” are Obsidian, Pumice, 
Perlite, Pitchstone, and Tachylite; the “ Crystalline” eruptive 
rocks are the Granite group, Felstone, Syenite, Trachyte, 
Phonolite, Andesite, Porphyrite, Diorite, Diabase, Gabbro and 
Basalt. The four last of these used to be called “ Greenstone” 
by the field Geologist, but this is now a vague word, like the 
<‘ Grauwacke” of earlier workers in the field. 
A diagram here given by the author showed the two series, 
the Acidic and the Basic. The Acidic kinds of crystalline 
rocks were represented at the top of the scale by Granite, which 
passed by insensible gradations into those vitreous forms called 
Pitchstone and Obsidian, the former being well represented in 
Arran, spécimens of which were exhibited. The highest crys- 
talline example of the Basic series was Gabbro (or Euphotide), 
