284 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



gether 99.475 per cent of the crust, leaving only a trifle more than 

 one-half of 1 per cent for all the other oxides. 



Thus we see that in round numbers silica is the most abun- 

 dant, and constitutes about six-tenths (nearly two-thirds) of the 

 crust ; alumina is next — a very poor second — slightly more than one- 

 seventh; then the two iron oxides, together about one-twelfth; 

 lime, about one-twentieth; soda, about one twenty -fif th ; magnesia, 

 about one-thirtieth ; potash, about one thirty-third ; water and titan- 

 ium dioxide, each about one one-hundredth; phosphorus pentoxide, 

 about one three-hundredth, and manganous oxide about one eight- 

 hundredth, while carbon dioxide is about one one-thousandth. Each 

 of the others is notably less than one one-thousandth. It will be 

 observed that in the list, which includes all the constituents that may 

 be commonly determined in really good and complete analyses of 

 igneous rocks, neither copper, lead, tin, zinc, mercury, silver, gold, 

 platinum, arsenic, antimony, nor several other of the elements com- 

 monly used in daily life are represented. The only common metals 

 shown are iron, aluminum, manganese, and nickel. This is a rather 

 important point that will be adverted to later. 



In order to form an idea of the actual rock that a magma of this 

 average composition would form under normal conditions we must 

 calculate, from the data given by the analysis, the presumable actual 

 mineral composition, or the "mode," as it is technically called. 

 There are two general and important conditions controlling the 

 products of solidification that may be considered. The magma may 

 have solidified, at considerable depth, slowly and under great pressure ; 

 or it may have solidified, as a lava flow, on the surface; that is, 

 rapidly and under low pressure. The former would furnish what 

 is called a plutonic rock (as a granite or a gabbro), and the latter an 

 effusive one (as a rhyolite or a basalt) ; and the different conditions 

 of solidification would bring about certain changes in the mineral 

 composition of the resulting rock. 



Such a calculation leads to the following results, which are to be 

 considered as only approximately correct, as variations in the mode, 

 of slight extent but in different directions, may be brought about by 

 slight variations in the conditions of solidification. As a plutonic 

 rock the magma would form a so-called granodiorite ; that is, a rather 

 coarse-grained, noncrystalline rock, much like many granites (and 

 which would be commonly called a rather dark granite) , composed of 

 feldspar, quartz, hornblende, or biotite, and very small amounts of 

 magnetite and apatite. If it solidified under surface conditions, the 

 magma would form that most common kind of lava, an andesite, 

 rather fine-grained, light gray or pinkish, and showing small crystals 

 ("phenocrysts") of feldspar and either hornblende or pyroxene, 

 with perhaps a little biotite, in a dense " groundmass." Under the 



