400 II. S. WASHINGTON ISOSTASY AND ROCK DENSITY 



Collections of physical determinations of the specific gravities of rocks 

 would include those of many rocks containing glass, so that the average 

 specific gravity thus arrived at would be low. We know that the plutonic 

 rocks, those formed at considerable depths, do not contain glass; and, 

 furthermore, examination of a large series of determinations of specific 

 gravity shows that the specific gravity of effusive lavas is generally less 

 (and by a notable percentage) than the specific gravity of plutonic rocks, 

 such as granite and diorite. Consequently the introduction of specific 

 gravity determinations of such glassy or effusive rocks along with plutonic 

 rocks would introduce serious error for our purpose of comparison of the 

 density of the average rocks of different areas. 



For our purpose, it matters not what may lie below, so long as we know 

 nothing of the characters of the deep-lying matter. We can deal only 

 with rocks as we know them, and base our data on ascertained facts and 

 rock characters. As an illustration of the possibility of the introduction 

 of serious error into our data by the use of physically determined specific 

 gravities, let us suppose a case, actual examples of which might readily be 

 found. Consider two areas of the same altitude (and about the same 

 size), the one (A) showing only effusive igneous rocks, say rhyolites, 

 while the other (B) shows nothing but plutonic rocks, let us say granite, 

 the rocks of both being of the same general chemical composition. It is 

 quite obvious on very slight consideration that the average specific gravity 

 (determined physically) of the two will not be the same; that of A will be 

 notably lower than that of B. The two areas, therefore, might appear not 

 to conform to the law demanded by isostasy. On the other hand, if we 

 calculate the densities from average norms, we would eliminate the 

 physical differences caused by the one portion of magma solidifying 

 quickly at the surface and the other slowly at depth; the average norma- 

 tive densities of the two areas of like rocks and of like altitude would be 

 practically identical, which is, ex liypothesi, what is to be expected. It 

 must not be lost sight of that the columns with which we are dealing 

 extend far below the surface, so that surface lavas can not properly rep- 

 resent them. It is also true that even the rocks which we call "deep- 

 seated" do not come from very great depths, isostatically speaking; but 

 they are the only data which we have, and it seems the more rational pro- 

 cedure to adopt a method which will reduce all the physically discordant 

 characters to uniformity and to the same conditions as to pressure, rate 

 of cooling, and other controlling factors, rather than to use a method in 

 which this source of error is at a maximum. 



Two minor sources of error are the porosity and the weathered condi- 



