Petrology of the Lavas. 



343 



Or, 



An, 



6*12 "| 



19-46 ! 



6-98 f 



Sal 



An, 



19-46 ) 



Sal 



Lc, 



45-34 



Ne, 



12-78 \ 





Ne, 



12-78 J 





Lc, 



11-77 ) 



44-01 



Di, 



24-68 "J 





Di, 



16*46 "| 





01, 



997 1 

 8-35 ( 



44-52 



01, 



13-80 | 



Fern 



Mt, 





Am, 



io-37 y 



50-34 



11, 



1-52J 





Mt, 



8-35 | 









Pf, 



1-36 J 





CaC0 3 , 



8-08 





H 2 0, 



2-66 





H 2 0, 



2-66 





co 2 , 



3-56 





100-60 



100-57 



Both of these norms lead to the same sub-rang position III, 7, 

 3, (3) 4, etindose. The ratio of salic K 3 0' to salic Na 2 0' = 0-60 

 so that it is on the exact line between sub-rangs 3 and 4 ; sub- 

 rang 3 has not yet, so far as the writer is aware, been named, 

 and the proper double name cannot, therefore, be given. In 

 calculating B the formula 3CaO, 2Si0 2 , recently established by 

 the Geophysical Laboratory of the Carnegie Institution, for 

 akermanite has been used. Akermanite is, of course, the 

 essential basis of the melilite molecule, and in B it appears, 

 due to the presence of the full amount of lime, diopside being 

 correspondingly reduced in amount on account of the defici- 

 ency in silica. In neither the basalt nor the monchiquite has 

 the anorthite been seen in the form of feldspar ; it is, there- 

 fore, in the augite, not, of course, as CaAl 2 Si 2 8 , but as CaAl 2 - 

 Si0 6 , Tschermak's augite molecule, and this helps to explain 

 the larger amount of this latter mineral actualty in the mode 

 than is shown in the norms. 



In regard to the lencite in the norms this is actually, in part 

 at least, embodied in biotite in the mode of the monchiquite, 

 with some of the olivine aiding it. It cannot be positively 

 stated that there is no leucite modally present in these rocks, 

 for lacking the usual twinning striations and characteristic 

 inclusions it could scarcely be distinguished from analcite. 

 None, however, has been observed. 



The Altered Zone of Soils and Gravels. 



The material of the samples of the well from 700 feet 

 upward has been to some extent also studied in thin section, 

 but without affording any additional information of especial 

 interest, partly because of the more or less profound alteration 

 it has undergone and partly because it appears, certainly for 

 the most part, composed of the same types of rocks as the 

 underlying lavas in place. This of course is what should be 

 expected from geological considerations, since the sedimentary 

 deposits represent those portions of the island which projected 



