1. The Rocks of Monte Ferru. 525 



slightly lower magnesia and lime of the flow, which makes it a 

 little more salic, a feature expressed by its symbol. A striking 

 character of both is the great preponderance of ferrous over 

 ferric oxide (in marked contrast to the trachytes and phono- 

 lites), as well as to a less extent that of soda over potash. 

 They both greatly resemble the Arci basalt and some of those 

 of the recent cones. The resemblance is less close with the 

 sheet basalt, the difference in the amounts of silica and potash 

 and the relations of the iron oxides being marked. 

 The norms of the two Monte Ferru basalts are : 



A B 



Q 0-48 



Or 6-12 6-12 



Ab _ 29*87 27*25 



An 22-80 18-35 



Di 7-88 15-48 



Hy 24-28 23-62 



01 _. 0-62 



Mt 1-16 1-86 



II _ 5-93 3-80 



Ap 1-34 0-67 



These norms place both rocks almost centrally in camptonose, 

 the symbol of A being "III. 5. 3. 4," and that of B being 

 III. 5. 3. 4". 



Mode. — It is scarcely practicable to measure the modes of 

 many of these basalts, partly because of the complexity of their 

 texture and partly because of the small size of many of the 

 mineral individuals. For the two rocks analyzed, however, 

 modes may be reckoned approximately from the norm, checked 

 as regards most of the minerals by study of their thin sections. 

 These yield the following figures, which can be regarded as, at 

 best, but rough approximations : 



A B 



Quartz 5 4 



A^An, __ 57 50 



Pyroxene 21 32 



Olivine _ _ 10 S 



Ores 6 5 



Apatite _ 1 1 



The presence of quartz, even though in small amount, in 

 these rocks along with olivine, and when it is not visible 

 in the thin section, will doubtless be surprising. Its modal 

 presence is, however, made necessary by the considerable 

 amount of actual olivine in the rock, while the norm shows 

 only hypersthene or but a trace of olivine. The crystallization 



