PETEOGEAPHICAL CHARACTERS OF MANSFIELD SLATE. 



59 



in which no anorthoclase replaces the orthoclase. As a consequence of 

 the easier solubiHty of the soda, this relation between the two alkalies, 

 soda and potassa, is maintained, and is often made more striking in the 

 clay slates. An average of 31 analyses of clay slates taken from varioiis 

 sources shows two and one-half times as much potassa as soda. In the case 

 of the Mansfield slate this difference has been increased, so that there is 

 ten times as nuich potassa as soda present. 



ANALYSIS OF MANSFIELD SLATE. 



Mr. George Steiger, of the United States Greological Survey, has 

 prepared a complete analysis (No. 1 in the following table) of a typical 

 specimen of the Mansfield clay slate. Analyses Nos. 2 and 3 were pre- 

 pared by W. Maynard Hutchings,' and numbered by him Nos. 2 and 5, 

 respectively. 



Analysis of the Mansfield clay slate. 



COMMENTS ON ANALYSIS. 



That which is the most striking about the analysis is the relative pro- 

 portion of the alkaline earths, lime, and magnesia, the latter being present 



'Notes on the composition of clays, slates, etc., ami on some points in their contact metamor- 

 phism: Geol. Mag., Vol. I, 1894, p. 38. 



