PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 133 



This analysis differs from the one actually performed only in fractions 

 of percentages in the alumina and soda, and I think may be assumed 

 as being nearly correct. The amount of iron in the two analyses is 

 identical, but there is some difference in the state of oxidation, which 

 can readily be supposed to be the result of secondary actions that have 

 taken place in the rock analyzed. 



It becomes very easy now to see how extremely diversified the feld- 

 spathic element may be in rocks of this nature. The molecules may 

 arrange themselves in very diversified ways, while the rocks remain 

 identical in composition. Circumstances of cooling might cause anor- 

 thite to separate in a nearly pure condition, when there would be a com- 

 pensating acidity in the remainder of the feldspathic element. This is 

 a much more satisfactory exijlanation than that which I offered at the 

 time I demonstrated the presence of anorthite in the West Eock dia- 

 base,^ for as my analysis showed, the presence of the anorthite did not 

 modify the ultimate composition of the rock. On the other hand pure 

 anorthite might be entirely absent and its molecules might enter into 

 combination with the molecules of the potassium and sodium feldspars, 

 to form one or more intermediate species, as in the Jersey City diabase, 

 and much diversity might exist in this feldspathic element in different 

 localities without the slightest change taking place in the ultimate com- 

 position of the rock. 



I regard this work as of some importance, since it completes our 

 knowledge of the normal composition of a rock which has a great dis- 

 tribution and very uniform characters, and shows that this rock is more 

 complex in composition than had been supposed. Besides it has been 

 common to consider what feldspar enters into the composition of basic 

 rocks like this, rather than what feldspars. An exquisite balance of 

 composition and circumstance would be necessary to crystallize such a 

 rock with a single feldspar, and we have reason to be convinced that 

 massive rocks are rarely simple as regards their feldspathic constituent.^ 



It has also an important bearing upon the microscopic determination of 

 feldspars by means of optical properties. The method proposed by 

 Pumpelly, and further developed by Fouque and Levy, is used for the 

 determination of the species of feldspar by seeking for the greatest 

 angles which elasticity planes make with twinning planes, in the zone 

 with axis perpendicular to the twinning plane. There is of course a pos- 



1 Am. Jour. Sci. 1875, vol. ix, p. 189. I concluded that a minute change in the com- 

 position of the rock would be sufficient to allow of the formation of anorthite, which 

 on account of its infusibility would first crystallize from the rock mass, as was evi- 

 dently the case. 



* Fouqud has demonstrated the complexity of the feldspar in Sfiutorin andesites, 

 Santorin et ses Eruptions, page 366. 



I have examined the basic rocks at Peekskill, on the Hudson, recently described by 

 Professor Dana, American Journal of Science, vol. xx, page 194. The feldspathic ele- 

 ment was easily separated into two parts, one of which was a pink variety of ande- 

 iste and the other white orthoclase. 



