NORMATIVE DENSITIES AND ALTITUDES 399 



ing to Benard, 21 the igneous rocks of these islands are largely granitic, 

 and we know that they formed part of the 9k! Archiplata shield. 



The physical Average Density 



The so-called physical method of obtaining the average density of a 

 given area is less complex and more direct than the normative method 

 which has been described in the preceding pages. Notwithstanding this, 

 there are such serious objections to it that the description and discussion 

 of it need not detain us long. 



According to this method, the available determinations of the specific 

 gravities of the rocks of the area are collected. It matters not whether 

 they have been analyzed or not, nor, if they have been analyzed, whether 

 the analyses are good. Their average is calculated by dividing their sum 

 total by the number of determinations. The specific gravity determina- 

 tions and the resulting averages may be grouped into separate areas or 

 not, as may be desired for the object in view. 



This method is, obviously, of great simplicity, but it would seem that 

 it has never been carried out, at least on a large scale. At first glance, it 

 may appear to lead to more reliable and more consistent results than the 

 normative method. It might be urged, for instance, that in using this 

 method we deal with the actual rocks and with their specific gravities, as 

 directly determined by the balance, whereas by the other method we deal 

 with hypothetical mineral molecules and arrive at our results only by a 

 very indirect route. But the case is not so simple as may be thought, and 

 the physical method is subject to several disturbing factors which may, 

 and usually will, introduce errors of very appreciable magnitude, if we 

 are to use the results in the comparison of the densities of different areas 

 or of rock masses of large size and considerable depth. 



One important source of error is the presence of more or less glass in 

 many lavas, some of them being wholly glassy. The glass form of a rock 

 or of a mineral has a density less by a very considerable percentage than 

 the crystalline form. To give an idea of the magnitude of this difference 

 the following figures may be cited from determinations made in the Geo- 

 physical Laboratory and published in various papers issuing from it : 



Specific Gravities 



Crystalline Glass 



Quartz 2.654 2.213 



Albite 2.605 2.382 



Anortliite 2 . 679 2 . 533 



Diopside 3.275 2.830 



Diabase 2 . 975 2 . 763 



- 1 A. Renard : Petrology of oceanic islands, 1889. 



