Febeuabt 5, 1909] 



8CIENGB 



205 



ically unlike parts and the resulting dif- 

 ferent varieties of igneous rocks, together 

 with their eruption and solidification as 

 geological bodies of various kinds. 



Assuming a certain elementary acquaint- 

 ance with rocks on the part of the student, 

 which is acquired in courses in general geol- 

 ogy, the systematic treatment of the subject 

 should begin by calling attention to the chem- 

 ical composition of unaltered igneous rock 

 as shown by analyses, published in many 

 descriptions of rocks, but most conveniently 

 found in comprehensive collections in Bul- 

 letins of the U. S. Greological Survey and 

 in- the tables of analyses edited by Justus 

 Roth and more recently by H. S. Wash- 

 ington. The extremely variable nature of 

 these data and their great abundance pre- 

 sent such an exceedingly complex set of 

 numerical relations that their statement, or 

 discussion, requires the aid of diagrams by 

 which the problem may be greatly sim- 

 plified. 



In addition to the chemical elements 

 noted in ordinary rock analyses there is a 

 much greater number known to occur in 

 rare minerals that crystallize from rock 

 magmas in special instances, or that oftener 

 appear in certain varieties of igneous rocks, 

 such as the pegmatites. A consideration 

 of all known pyrogenetie minerals with 

 respect to their chemical composition calls 

 attention to the compounds that are re- 

 peatedly formed in igneous magmas by the 

 union of the elements that existed in the 

 magmas before their solidification. And by 

 arranging them in accordance with the 

 order of their constituent elements in the 

 Mendeleeff series valuable information as 

 to certain ©hemical relationships among 

 these compounds is at once furnished. 



The substances occurring in igneous rocks 

 are in most eases solids, less often liquids or 

 gases. The solid compounds are always in 

 crystallized condition. Amorphous, glassy 

 solids that sometimes occur in igneous rocks 



are seldom, if ever, definite chemical com- 

 pounds, but are mixtures. The crystallized 

 substances (minerals) are rarely present as 

 uncombined elements, such as gold, graph- 

 ite and metallic iron. A few are simple 

 compounds with invariable composition, as 

 SiOg (quartz), TiO^ (rutile), ALOj (cor- 

 undum). Most of them are complex and 

 variable in composition, owing to the pres- 

 ence of isomorphous mixtures, as the feld- 

 spars, olivine, amphiboles. There are very 

 few examples of polymorphism, such as 

 quartz and tridymite. The apparent dif- 

 ference in the crystallization of orthoclase 

 and microcline is probably due to submicro- 

 seopic multiple twinning in the apparently 

 more symmetrical form. Polymorphism of 

 some of the pyrogenetie compounds, as 

 MgSiOa and CaSiOa, which is known in 

 laboratory products is not clearly de- 

 veloped in pyrogenetie minerals. 



Certain isomorphous mineral compounds 

 are not developed in igneous magmas with 

 like frequency, or in certain cases not at 

 all. Such, for example, are the hexagonal 

 compounds NaAlSiOi (sodium-nephelite), 

 KAlSiOi (kaliophilite), LiAlSi04 (eucryp- 

 tite). Compare also the potash-, lithia- 

 and soda-micas. Other compounds that are 

 analogous chemically and might be expected 

 to crystallize isomorphously in igneous mag- 

 mas have quite different crystal symmetry, 

 as is the case with KAl(Si03)2 (leucite), 

 NaAUSiOs)^ (jadeite), LiAKSiOs), (spo- 

 dumene). 



Various silicate compounds involving dif- 

 ferent silicic acids : orthosilicic, metasilicic, 

 polysilicie and in rare instances disilicic, 

 besides uncombined silica, may crystallize 

 from the same rock magma. And even 

 base-forming elements, as iron and alumin- 

 ium, may, under some conditions, separate 

 from rock mag-mas as oxides without com- 

 bining with silica, which may itself sepa- 

 rate as SiOo. That is, hematite, magnetite 

 or corundum may crystallize in the pres- 



