Habit of Igneous Rocks

The question may be asked—When is a rock entitled to be recognized as belonging to a distinct species or variety and de serving a name for itself? It must, first of all, be proved to occur in considerable quantity at some locality, or better still at a series of localities or to have been produced from different magmas at more than one period of the earth's history. In other words, it must not be a mere anomaly. Moreover, it should have a dis tinctive mineral constitution, differing from other rocks, or some thing individual in the characters of its minerals or of its struc tures. It is often surprising how peculiar types of rock, believed at first to be unique, turn up with identical features in widely scat tered regions; alnoite, for example, occurs in Norway, Scotland, Montreal, British Columbia, New York and Brazil, tinguaite in Scotland, Norway, Brazil, Montana, Portugal, etc. This indicates that underlying all the variations in mineralogical, structural and chemical properties there are definite relationships which tend to repeat themselves, producing the same types whenever the same conditions are present.

Various attempts have been made to introduce

a quantitative element into igneous rock classification. These have followed either chemical or mineralogical lines. The principal of these is the Quantitative chemical classification introduced by a group of American petrographers. The chemical composition of an igneous rock is here regarded as its fundamental characteristic and a series of subdivisions is erected on this basis. Other criteria are rele gated to the background. The completed rock analysis is first interpreted in terms of an ideal set of minerals which constitute the "norm," but which in reality seldom corresponds to the actual composition ("mode"). The rocks are then divided into groups strictly according to the relative proportions of these ideal min erals to one another. The details of the classification need not be described here as they are fully set forth in a treatise specially devoted to this classification (Quantitative Classification of Ig neous Rocks, Chicago, 1902 ; see Bibliography).

In other systems a quantitative element is introduced by sub dividing rocks into groups by arbitrary lines based on mineral percentages : a complicated system framed on these lines is that devised by A. Johannsen (Journal of Geology, 1917, 1919). In another the prime divisions are erected on a so-called "principle of silica-saturation" (Shand, Eruptive Rocks, 1927). In all the effect of the introduction of the quantitative element has been to increase the artificiality of the system and by this means to obscure the genetic relations existing between rock types. For this reason these systems are unlikely to supplant the qualitative clas sification in general use. It is indeed, largely to the influence of "quantitative classifications" that the bewildering and unneces sary multiplicity of igneous rock names must be ascribed.

A natural classification based on the genetic relationships of the igneous rocks is yet to be framed. The writings of Becker and Vogt contain some essay towards this goal. As a basis, the eutectic relation has been emphasized, the eutectic mixture pos sessing definite properties and a fixed composition. Experimental work of recent years has clearly shown however that the eutectic relation is only one of a number of possible relations existing between the components of a magma ; that its importance has been over-emphasized is abundantly shown by the experimental investigation of silicate melts, and is indeed revealed in the se quence of crystallization decipherable in igneous rocks.

Petrographical Provinces.

As long ago as 1872 Vogelsang had noted that the igneous rocks of certain districts possessed textural or mineral characters in common, serving to distinguish them from the rocks of other districts. The researches of Judd (i886) and Iddings (1892) further substantiated this observa tion and showed that this community of petrographic character applied to the igneous rocks erupted or intruded during a particu lar period of igneous activity within the region. To such regions Judd gave the name petrographical provinces and Iddings em ployed the term consanguinity to express the genetic relationship implied in these resemblances among associated rocks. Excellent examples of such petrographical provinces or comagmatic regions, as they have been termed by Washington, are the Oslo region of southern Norway characterized by Devonian alkaline intrusions and extrusions ; the Roman region characterized by rocks rich in potash; the alkaline province of central Montana; and the Ter tiary calc-alkaline province of Hungary. On a larger scale the volcanoes which girdle the Pacific (Andes, Cordillera, Japan, Phil ippines, etc.) and those which occur on the volcanic islands of the Atlantic, illustrate the same phenomena. The consanguinity in the igneous series of a petrographical province implies that the whole assemblage is derived from some common deep-seated magma, during a period which, while necessarily prolonged, was not of vast duration in a geological sense. The assemblages of a province may and often do show a wide diversity of rock type embracing intrusions and extrusions. Prolonged eruptions have in a few cases a somewhat monotonous character owing to the predominance of one kind of rock. Thus the lavas of the Hawai ian islands are mostly basaltic, as are those of Oregon, Washing ton and the Deccan, all of which form geological masses of enor mous magnitude. In the Oslo district on the other hand the assemblages comprise (among the intrusive rocks) a wide range of type—hornblendite, essexite, larvikite, lardalite, nordmarkite, soda—and potash-granites.