132 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



tiDjiuishable liuder the microscope, and the optical properties of the 

 grains otiered uo peculiarities to conflict with the above determination. 



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

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

 quite comi)lete 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 liis analysis was made on more hydrous material; there- 

 fore I will use my old analj'sis 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: 



Si02.... 

 AI2O3... 

 re203.. 

 FeO.... 

 MnO... 

 CaO..., 

 MgO .. 

 Na20 . . . 

 £20... 

 Ti02... 

 P2O6... 

 Igniton 



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

 vol. ix, 1875, page 185. ♦ 



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

 late api^roximately the jjercentage com]>osition 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, 22.1C; potash feldspar, 2.32 ; augite, 54.47; 

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



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

 as follows : 



Si02 51.78 



AI2O3 12.46 



FeaOa 92 



FeO 10.67 



JNlnO 44 



CaO 10.77 



MgO 7.63 



Na,0 2. 62 



K>0. 



TiO., 

 P2O5 

 H2O. 



39 



.41 



14 



03 



99.86 



