

40 



in different proportions. (1) The origin of these patches cannot be explained 

 in all cases ; sometimes they may indicate an imperfect mixing of the original 

 constituents of the rock a want of homogeneity in the original magma ; at 

 other times they may be due to the metamorphism of included fragments of 

 foreign rocks, and their injection by the igneous magma ; and at others to 

 a separation of the minerals first formed by gravity or other causes. 



Any want of homogeneity in the original magma must of course tend 

 to produce a want of homogeneity in the volcanic rocks produced by its 

 eruption at the earth's surface in the form of lava. The differential motion 

 of the lava will tend to drag out any parts of exceptional composition 

 into the form of streaks or elongated lenticles (schliere). (2) We accordingly 

 find that a certain want of uniformity is a common characteristic of many 

 volcanic products ; especially those which are rich in silica. 



The bulk-analysis of an igneous rock may be regarded from two points 

 of view: it may be considered with reference to the analyses of the 

 individual constituents of the rock itself, or with reference to the bulk- 

 anajyses of other rocks. In discussing the analyses of rocks and minerals, 

 the oxygen-ratio (quantivalent ratio of modern chemistry) and oxygen- 

 quotient are of considerable utility. For the purpose of ascertaining the 

 oxygen-ratio, the constituents are divided into three classes, in consequence 

 of the fact that the members of these classes are capable of replacing each 

 other to a certain extent in the rock- forming minerals without affecting the 

 crystalline form. Silica and titanic acid belong to the first class; alumina 

 and ferric oxide (the sesquioxides) to the second ; and the remaining 

 constituents (the monoxides), lime, magnesia, ferrous oxide, and the alkalies, 

 to the third. The symbol R stands for a double molecule of one of the 

 metallic elements in a sesquioxide base (i.e., for A1 2 , Fe 2 , or Cr 2 ), and R for 

 a molecule of a metallic element in one of the monoxide bases. R may 

 therefore represent a molecule of a dyad element, such as calcium, magnesium, 

 or ferrous iron ; or that of a monad element, such as potassium, sodium, or 

 lithium. It is sometimes convenient to indicate the valency of the element 

 for which R stands, by a sign over the R, thus: it represents a monad 

 element, such as one of the alkalies ; E, a dyad element, such as calcium or 

 magnesium. Adopting these symbols, the common bases of the minerals of 



VI II I 



igneous rocks may be expressed as follows: KO, no, and R 2 o. In deter- 

 mining the oxygen-ratio, no and no are usually taken together. This ratio 

 is the ratio of the amount of oxygen combined with the silicon to that 

 combined with the elements represented by R, to that combined with the 

 elements represented by R. Thus, in the following analysis of granite, by 

 Dr. HAUGHTON, we have 



(2) PHILLIPS, J. A., On concretionary patches and fragments of other rocks contained in 

 Granite. Q.J.G.S., Vol. XXXVI., p. 1. 



(2) REYEB, E. Vulkanologische Studien. Jahr. d. geol. Reichsanstalt. Vol. XXVIII., p. 81. 

 Vienna. 



Die Euganeen. Wien, 1877, p. 69, and Vol. XXVIII., p. 216. 



