522 Washington — Contributions to Sardinian Petrography: 



the former, as seen in thin sections, contains decidedly more 

 aegirite than that of the latter rock. The designation of tra- 

 chytic phonolite is fully justified by the analyses and their 

 norms. 



Basalt ( Gamptonose). 



The basalts of Monte Ferru are divided by Doelter into 

 olivine-free and olivine-bearing varieties. Dannenberg and 

 Deprat, on the other hand, lay no stress on the presence or 

 absence of olivine, and refer them to two groups : andesitic 

 basalts, gray, rather coarse-grained and with feldspar pheno- 

 crysts, which form tabular masses on the higher parts of the 

 mountain ; and basalts proper s dark and fine-grained, without 

 feldspar phenocrysts, which form the flows on the lower flanks 

 of the cone and extend far beyond in all directions. The 

 former issued from large dikes, with a general north-south 

 trend, which are met with in the central parts of the volcano ; 

 and the latter from more irregular dikes lower down on the 

 flanks. The reader may be referred to Dannenberg* for 

 descriptions of these occurrences. 



I have made only two analyses of these rocks, which are 

 much alike, and several more are needed for their proper 

 study. Practically all the basalts collected by me carry more 

 or less olivine, and these differ chiefly in the size of the feld- 

 spar laths of the groundmass. I have but few specimens of 

 the "table-basalts" of Dannenberg. These are rather light in 

 color and show feldspar phenocrysts, in accordance with the 

 distinction made by Dannenberg and Deprat. But, on the 

 whole, the difference between them does not seem to me to be 

 of great petrographic significance, so that, in the absence of 

 sufficient analyses and of a more detailed study of their various 

 occurrences, it seems to be advisable at present to make no 

 marked distinctions and to describe them together. 



Description. — The basalts vary in color from a rather light 

 to a very dark gray. They are all fine-grained, some, espe- 

 cially those forming the dikes, being very dense and aphanitic. 

 They are only rarely vesicular, but never scoriaceous. 

 In the lighter colored specimens, as from near Monte Entu 

 and Nuraghe Sylvanis, there are some phenocrysts of feld- 

 spar (from 2 to 5 mm in diameter), and in the specimen from 

 the latter place are also phenocrysts of black augite and 

 olivine, these rocks being almost dopatic. Nearly all the 

 basalts, however, are almost or quite perpatic, small pheno- 

 crysts of augite or olivine being rare. 



In thin section they are seen to be fairly uniform. Plagi- 

 oclase, with an average composition of labradorite, about 



* Dannenberg, Neues Jahrb., B. B., xxi, p. 38, 1905. 



