6 14 HENRY S. WASHINGTON 



It will be seen that, though the covite and the theralite of 

 Wolff resemble each other in qualitative mineralogical compo- 

 sition, as both are composed essentially of alkali-feldspar, nephe- 

 lite and ferromagnesian minerals, and that both are distinctly 

 leucocratic in character, yet that in a quantitative mineralogical 

 way they are decidedly different. The feldspathic constituents 

 of the covite are very largely feldspar, with only accessory 

 amounts of nephelite, while the theralite shows about as much 

 feldspathoid as feldspar. The calculation of the latter cannot 

 be exact, since some of the soda goes into the feldspar, but this 

 must be small, and cannot affect the result to any great extent. 

 It is evident, then, that the name of theralite is not appropriate 

 for the Magnet Cove rock, though it might be used in the 

 present very vague and loose method of classification, based 

 largely on qualitative mineralogical composition. 



A comparison of Pirsson's descriptions ^ with Rosenbusch's 

 definition of shonkinite indicates that the latter has been appa- 

 rently laboring under a misapprehension of the former's descrip- 

 tions, and that his definition does not cover the rocks as Pirsson 

 described them. Pirsson expressly states in each case that 

 nephelite is either entirely absent or present only in mere 

 traces, which does not coincide with the definition which makes 

 nephelite an essential constituent. 



Although resembling each other in many ways, yet there are 

 certain striking differences between the analysis of the Magnet 

 Cove rock and those of shonkinite. In SiOg, iron oxides, CaO and 

 KgO they are closely alike, but in the Magnet Cove rock AlgOg 

 and Na^O are higher and MgO lower. Indeed the calculations of 

 the mineralogical composition, though that of la is only approxi- 

 mate, show clearly that while the " covite " is distinctly leuco- 

 cratic the shonkinite is as decidedly melanocratic. A similar 

 distinction will be pointed out between the " leucite-porphyry " 

 and missourite. In this respect the rock under consideration 

 resembles the typical essexite, though here again there is a dis- 

 tinct difference in the amount of K^O, in the essexite this being 



'L. V. Pirsson, Bull. Geol. Soc. Am., Vol. VI, p. 408, 1895; Am. Jour. Sci.,No\. 

 L, p. 474, 1895 ; Am. Jour. Set., Vol. I, p. 358, 1896. 



