648 HENRY S. WASHINGTON 



physical research, and this point is mentioned only because such 

 methods have, as yet, found little application in petrography. 



In order to construct the curves two important field data are 

 required ; the position of the center of the mass or area, and the 

 relative distances from this of the various types analyzed. 



By center is meant that of the innermost petrographic zone 

 or core, not necessarily the geometrical center of the mass, as 

 this petrographic center may be conceivably geometrically 

 eccentric. Since, in many cases, as here, we have only one sec- 

 tion, a horizontal one, this point is not necessarilv the center of 

 the mass, but rather its epicenter, to use the seismological term. 



Since the area of Magnet Cove forms a fairly regular ellipse 

 with axes of about 5 and 3 kilometers, the center of the igneous 

 area is easily determined. Its position is marked approximately 

 by the Baptist church,^ which lies in the small central exposure 

 of biotite-ijolite. Inasmuch as we do not know whether the 

 plane of the present exposed area cuts the mass centrally or 

 above the center, we cannot tell whether the central point of 

 this is the true center of differentiation or not. It is probably 

 not so. But as far as the types exposed are concerned this is of 

 little moment, as their mutual relations would remain the same, 

 or approximately so, in any case. 



Having determined the center the next point is to determine 

 the distances of the various types from this. It is obvious that 

 for the proper plotting of the curves, and hence the study of the 

 course of differentiation, this is of great importance, since the 

 points which determine the various curves will be shifted in one 

 direction or another according to the distances selected. This 

 would alter very materially the slope of the curves, and even 

 their, character or shape, as by the shifting of the abscissal posi- 

 tions a straight line will become a curve, or a simple curve of the 

 second degree may assume the form of an inflexed one of the third. 



At Magnet Cove we cannot determine the abscissal positions 

 by simply measuring the distances from the center to the par- 

 ticular spots where the analyzed specimens were collected, 



' Cf. the maps in papers already cited. 



