6o8 HENRY S. WASHINGTON 



fraction of I per cent, can be deduced from the diagram." This 

 instance is the first in which the differentiation of a mass of 

 magma is rendered capable of exact mathematical treatment and 

 proof, and its great theoretical interest and importance is obvious. 

 The conditions of the locality were such that it was thought that 

 the Magnet Cove complex might furnish another favorable 

 example of the same kind, a hope which was justified by the 

 results obtained, as will be seen later. 



The analyses of six of the representative plutonic rocks of 

 Magnet Cove, as well as two of Fourche Mountain, were there- 

 fore undertaken, with the determination of the rarer constituents 

 which might be present. For petrographic descriptions the 

 reader is referred to the two papers cited above. 



In this connection I would express my full endorsement of 

 Pirsson's remarks^ on the importance of good and complete 

 analyses, which are absolutely essential for such mathematical 

 discussion, and with him deplore their comparative rarity. To 

 many petrographers, any collection of figures which foots up 

 within 98 and 102 per cent, is a usable analysis, even though the 

 results are at variance with the mineralogical composition, and 

 some of the obviously important constituents are not estimated. 

 It is not realized that such "analyses" do far more harm than 

 good to the science. From this point of view alone, the excel- 

 lent work of Dr. Hillebrand and the other chemists of the United 

 States Geological Survey is of inestimable value, as they have 

 set a standard to fall short of which in any marked degree 

 should be accounted a petrographic sin. Let me be the first to 

 confess ^' peccavi." 



ANALYSES. 



Pulaskite. — The specimen of this type was collected at the 

 Little Rock Granite Company's quarry, at Fourche Mountain, 

 near Little Rock, the type locality. In I is given my analysis, 

 in II that of R. N. Brackett, as quoted by Williams. The two 

 do not differ materially, I showing rather less FegOg and CaO 

 and more FeO, MgO, and alkalis, though the ratio of 



' L. V. PiRSSON, op. cit., p. 578. 



