132 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



tinguishable under the microscope, and the optical properties of the 

 grains offered no peculiarities to conflict with the above determination. 



The analysis of the anorthite and augite that I picked from West 

 Rock may be added, and our knowledge of this diabase may be said to be 

 quite complete as regards the composition of the fresh rock. I will place 

 together the analyses of the rock and its other components. Professor 

 Genth's analyses, to which I have referred, is more complete than any 

 that I have made, since he determined the traces of lithia, copper, and 

 sulphur. But his analysis was made on more hydrous material ; there- 

 fore I will use my old analysis of West Eock, New Haven, because the 

 analyzed material was very fresh, bright, and clear, and also illustrates 

 the commonest variety of the rock. 



This rock and the following minerals from it have been analyzed : 



* My analysis with the titanic acid determinations by Dr. A. B. Hotve, American Journal of Science, 

 voL ix, 1875, page 185. 



Knowing that the feldspathic element is complex we can now calcu- 

 late approximately the percentage composition upon the basis of the 

 elements which are peculiar to the species involved. If in this manner 

 neglecting the water we determine the percentage of the mineral con- 

 stituents, we obtain 



Anorthite, 15.52; albite, 22.16; potash feldspar, 2.32 ; augite, 54.47; 

 titanic iron, 2.68; magnetite, 1.76; apatite, .32; total, 99.23. 



The composition of this mixture, on adding again the water, would be 

 as follows : 



SiOz 51.78 



AI2O3 12.46 



FezOa 92 



FeO 10.67 



MnO 44 



CaO 10.77 



MgO 7.63 



NaKjO 

 K2O.. 

 TiOa . 



P205 , 

 H20.. 



2.62 

 .39 



1.41 

 .14 

 .63 



99.86 



