390 /. MORGAN CLEMENTS 



would hardly be warranted. As has already been said, how- 

 ever, the rock of which the analysis was made is one which is 

 feldspathic, and represents a transition upward into the gabbro. 



Examining the series of analyses together, we see that in 

 passing from the most acid to the basic end the alumina increases 

 very rapidly to 25.96 per cent., until it reaches the extreme basic 

 rock, when it drops suddenly to 6.22 per cent. The analyses 

 also show an increase in the same direction in iron, as is best 

 brought out in Table III. The alkalies increase with diminishing 

 silica, whereas the magnesia, which for rocks of this character 

 is very characteristic, shows a decided increase. In the gabbro- 

 norite-peridotite portion of the series, analyses Nos. 2, 3, and 4, 

 the lime shows a constant diminution, corresponding to the 

 increasing magnesian character of the rocks. Likewise the pot- 

 ash increases as the soda diminishes. The rocks represented by 

 the analyses are believed to belong to a series ranging from a 

 diorite on the one hand through hornblende-gabbro and norite 

 to a peridotite on the other. It is evident that a gap exists 

 between the gabbro and diorite. The diorite represents a grada- 

 tion towards the orthoclase rocks of essentially the same acidity. 

 On this acid side of the series the microscope also shows varia- 

 tions to a tonalitic and even granitic rocks very rich in quartz 

 and orthoclase, probably very much more acid in character than 

 the diorite represented in the analyses. 



Relations of the rocks of the series. — The rapid changes in min- 

 eralogical composition and texture in a single rock exposure, and 

 the changes thus occasioned from one type into another through 

 intermediate facies, show very clearly the intimate relationship 

 of the Crystal Falls rocks to one another, and warrants the 

 assumption that they all belong to a geological unit, a conclu- 

 sion long since reached by Williams J for a similar group of rocks, 

 " The Cortlandt Series " from New York. Variations very similar 



1 The peridotites of the "Cortlandt Series " on the Hudson River near Peekskill, 

 N. Y., by G. H. Williams, Am. Jour. Sci., Vol. XXXI, 1886, pp. 26-41. 



The norites of the " Cortlandt Series " on the Hudson River near Peekskill, N. Y., 

 by G. H. Williams. Am. Jour. Sci., Vol. XXXIII, 1887, pp. 135-144, 191-199. 



