FOYAITE-IJOLITE SERIES OF MAGNET COVE 657 



and Brazil, that in large steel castings, such as those for modern 

 artillery, etc., there is a very marked concentration of "impuri- 

 ties," as phosphorus and sulphur, toward the center of the mass. 



As all the curves are so smooth and well defined, it seems 

 highly probable that equations for them could be found and that 

 their properties as such could be discussed. In this way we 

 could cret at an exact knowledge of the law of differentiation, 

 in this particular case at least. It is a matter of regret that I 

 am not mathematician enough to do this, but there are other 

 applications of the data at hand which are capable of simple 

 mathematical treatment. 



Since the area of Magnet Cove is a fairly regular ellipse, and 

 the zones of the various types are concentric about the center, 

 by taking the average distance of each we practically reduce the 

 ellipses to circles, the average distances being the radii. 



Now, since II is at the center, if we suppose Diagram 3 to 

 be revolved about the vertical line at II as an axis, it follows that 

 the solids of revolution so generated by each of the curves (with 

 the bounding lines at the sides and bottom), will represent the 

 amount of each oxide in the orig^inal maa^ma, and that their sum 

 will represent the composition of the magma as a whole, before 

 differentiation. 



This is not strictly true, since we are ignorant of the exact 

 shape and extent of the complex, but as a first approximation 

 and an illustration of the method, it will be of interest to calcu- 

 late the results which are obtained on this basis. As a matter of 

 fact, the recent description of the Shonkin Sag laccolith by 

 Weed and Pirsson^ renders it extremely probable that the foyaite 

 is present in far greater relative amount than the surface expo- 

 sures indicate. This would necessitate a very considerable cor- 

 rection, but, as we have no means at present of estimating this, 

 it will be as well to give the figures based solely on the field 

 observations, leaving possible corrections for the future. 



The process of calculating the various volumes is very simple 

 in theory, but somewhat complicated and laborious in practice. 



^Weed and Pirsson, Am. Jour. Sci., Vol. XII, p. i, 1901. 



