214 



SCIENCE 



[N. S. Vol. XXIX. No. 736 



liquid, composition of the liquid, tempera- 

 ture, pressure and time, or rate of changing 

 conditions. The combination of these fac- 

 tors in the case of any cooling rock magma 

 results in rock possessing a certain degree 

 of crystallinity, which may range from a 

 state of complete crystallinity, to the re- 

 verse, or complete glassiness. When more 

 or less crystalline, the size of the crystals 

 becomes a feature of consequence. The 

 granularity, or the size of crystals in rocks, 

 has been given a prominent role in most 

 descriptions, and classifications of rocks. 

 The shapes of individual crystals clearly 

 give distinctive character to the pattern, 

 or fabric, of rocks, and shape is largely a 

 function of crystal structure and physical 

 habit of specific minerals. The recognition 

 of these relationships and their systematic 

 treatment in the description and discussion 

 of igneous rocks will lift the subject out of 

 a maze of confusing, complex detail, usu- 

 ally treated in an uncoordinated and mean- 

 ingless manner. 



Application of principles of molecular 

 diffusion ; of laws relating to solution pres- 

 sure, or osmotic pressure; of conditions 

 controlling crystallization, or the separa- 

 tion of solids from solutions ; of conditions 

 affecting the physical character of liquids, 

 or rock magmas ; to the observed variability 

 in the composition of igneous rocfe, and to 

 the known relation between their composi- 

 tion, order of eruption, and mode of occur- 

 rence, leads to conceptions of their origin 

 from other magmas, by processes called by 

 the general designation of differentiation. 



With such an understanding of the 

 causes of heterogeneity in rock solutions 

 the great variability in the composition of 

 igneous rocks as shown by chemical an- 

 alyses, and by a quantitative study of their 

 mineral composition, appears as the nat- 

 ural, as well as the logical, result of their 

 mode of formation. 



Mineralogical and constitutional facies 

 of igneous rocks are readily comprehended ; 

 and the absence of fixed types of magmas, 

 or of frequently recurring bodies of igneous 

 rocks with definite or invariable composi- 

 tion, becomes "natural," and is the thing 

 to be expected. Variations in texture 

 within one rock mass, and among rock 

 bodies having various modes of occurrence, 

 are readily understood as the results of 

 variability in the conditions attending vol- 

 canic eruption. 



As to the possible character of volcanic 

 eruption, some conception of it may be de- 

 rived from a consideration of the probable 

 condition of highly heated rock material 

 under great pressure deep beneath the sur- 

 face of the earth, as well as its probable 

 experience in moving upward and out upon 

 the earth's surface. 



The high temperature of volcanic lavas 

 when they reach the atmosphere, the fact 

 that they were losing heat continually from 

 the time of their first movement upward, 

 the evidence that they were completely 

 liquid at some stage in their eruption, to- 

 gether with the observed gradient of in- 

 crease of temperature downward from the 

 surface of the earth, all combine to show 

 that rock magmas come from some region 

 where the temperature is considerably 

 above the melting point of igneous rocks. 

 The behavior of the earth as a rigid globe, 

 and the known effect of pressure in counter- 

 acting that of heat, together with its esti- 

 mated high gradient of increase downward 

 within the earth, force the conclusion that 

 at sufficient depth magma, though hot 

 eno\agh to be liquid, behaves as a solid. 

 Such conditions of heat and pressure can 

 not vary abruptly from place to place, but 

 must be nearly the same for large volumes 

 of material ; and differences of temperature 

 and pressure must obtain very gradually, 

 chiefly in vertical directions. Magma in 



