EARTH'S CRUST — WASHINGTON. 271 



7 per cent, and the atmosphere at 0.03 per cent, of the complex crust. 

 In the following pages, however, the hydrosphere and the atmos- 

 phere and the. sedimentary rocks will not be taken into account, and 

 we shall consider the " crust " as made up wholly of igneous rocks. 

 This is the more justifiable for our present purposes, because the 

 material of the sedimentary rocks has been derived entirely, either 

 directly or indirectly, from preexisting igneous rocks, while the 

 metamorphic rocks (gneisses, schists, etc.) have been formed from 

 either igneous or sedimentary rocks. 



When we consider, then, onty the igneous rocks of the earth as a 

 whole, we know that they are not all alike, but show wide differences 

 in their characters, chemical and physical. There are here two main 

 questions regarding them to be considered. 



The first is: What is the average chemical composition of the 

 igneous rocks of the crust? The answer to this is of considerable 

 importance for the investigation of the constitution of the earth, 

 and is also of interest in the study of the rocks themselves — the 

 science of petrology. 



The second question is: Do the igneous rocks, taken as a whole, 

 show sensible uniformity as to general characters, or do they differ 

 noticeably in different portions of the earth's surface? That is, is 

 the earth's crust sensibly alike or unlike? 



Attempts to answer these questions, with some consequences that 

 seem to follow from their consideration, will form the chief topics 

 of this paper. 



GENERAL CHARACTERS OF IGNEOUS ROCKS. 



For present purposes we can not go deeply into the characters of 

 igneous rocks, nor discuss them all — a subject that has produced a 

 very voluminous literature. It is needful here to present only some 

 of the salient and pertinent facts. 



Igneous rocks, as has been said, are those that have solidified 

 from a state of fusion, or rather liquidity, as the term "fusion" 

 implies a previous solid condition. The liquid matter, that even- 

 tually solidifies as a rock, is called technically the " magma," a term 

 that is in frequent use in petrology. 



The magma comes up from below ; from what depth we do not 

 know, though there is some reason for thinking that the places of 

 origin are not very deep. Nor do we know whether it arises from 

 the melting of portions of the earth that are actually solid but 

 potentially liquid on relief of pressure, or whether it is, in general, 

 derived from " reservoirs " of liquid magma. 



