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The Classification of a Glassy Rock: The Pitchstone of Wormit, Fifeshire
Published online by Cambridge University Press: 01 May 2009
Extract
In most systems of petrography the glassy rocks are treated in a very casual way. The names which are commonly given to them, such as obsidian, pitchstone, perlite, vitrophyre, and the like, afford no reliable indication of composition; and when a rock is partly crystalline and partly glassy, the composition of the glass is often assumed, quite unwarrantably, to be the same as that of the crystals. It is only in the Norm classification that vitreous rocks fall unfailingly into the same compartments as holocrystalline ones of similar chemical composition; and before any rock can be classified by this method, it is necessary to have a complete chemical analysis of it; which is, for many workers, a serious obstacle.
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- Copyright © Cambridge University Press 1929
References
page 117 note 1 Eruptive Rocks, London (T. Murby and Co.), 1927.Google Scholar
page 117 note 2 Mineralogical Magazine, 1922, p. 275.Google Scholar
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page 119 note 1 Insets (my terminology) are the “phenocrysts” of those who prefer bobtailed Greek to plain English.