382 II. S. WASHINGTON ISOSTASY AND ROCK DENSITY 



men in air and water. This may be called the physical method. We shall 

 discuss the normative method first and at greater length, because it is the 

 more novel and, which is of more importance, it appears to lead to more 

 reliable and mutually consistent results than the other. 



THE AVERAGE ROCK 



As a fundamental basis for the normative method we must obtain a 

 knowledge of the chemical composition of the average igneous rock of the 

 area, whether it be a small district, State, country, continent, or the whole 

 earth. These average compositions have been arrived at by averaging the 

 figures presented in a collection of chemical analyses of igneous rocks 

 published between 1883 and 1913, inclusive. 6 This covers all the petro- 

 logically known parts of the earth's surface, and includes over 8,000 

 analyses of varying degrees of merit. These were divided into two groups 

 according to their quality — the superior analyses, which were considered 

 to be satisfactory as to accuracy and completeness, and therefore reliable 

 and sufficiently good for use, and the inferior analyses, which are either 

 so incomplete or so inaccurately made, er both, that they are excluded 

 from consideration for the present purpose. 



For making the averages, 5,159 superior analyses were used, those of 

 separate districts, States, or countries being averaged separately, and the 

 results were combined into averages for larger areas, continents, and 

 ocean floors, 7 and finally for the whole earth. The greater part of this 

 laborious series of computations was carried out by Dr. F. W. Clarke, to 

 whom I am deeply indebted for his invaluable cooperation. Some sup- 

 plementary averages have also been computed by'me. All these averages, 

 numbering about 50, are to be given and discussed at length in a forth- 

 coming professional paper of the United States Geological Survey, under 

 the joint authorship of Dr. Clarke and me. Some of them, which are best 

 illustrative of the relations of rock density to isostasy, are presented here, 

 and I have again to thank Dr. Clarke for kindly consenting to this prior 

 publication. 



Various objections have been and will be raised against averages ob- 

 tained in this way. One is that the actual relative masses of the different 

 kinds of rock are not properly taken into account, because there is an 

 unduly large proportion of analyses of interesting and rare (therefore 

 less generally abundant) kinds, while too few analyses are made of the 



8 H. S. Washington : U. S. Geol. Survey Professional Paper no. 90, 1917. 



7 The analyses of the rocks from the islands of the various oceanic basins were used 

 to determine the averages for the ocean floors, as these lavas came up from magmas 

 underlying the oceans, and they are the only means which we possess of obtaining the 

 knowledge required. 



