/. The Rocks of Monte Ferru. 519 



ble. They may, however, be approximately calculated from 

 the norms, the glass base in B being assumed to carry the 

 quartz and andesine. To the modal augite is assigned a little 

 normative anorthite, ilmenite and hematite. The results are 

 as follows : 



A B 



Quartz 2'0 25 



Or 7 Ab s 62-0 73-0 



A^An, 26-0 16-0 



Augite 5-0 3-5 



Ores 5-0 5-0 



100-0 100-0 



It will be seen that the mode of the Cuglieri rock is strictly 

 normative, while that of the other would be so were it holo- 

 crystalline. 



Trachytic Phonolite (Phlegrcse-beemerose). 



Occurrence. — The trachytes of the Sennariolo type pass by 

 imperceptible gradations, chiefly in the nephelite content, into 

 what may be considered to be phonolites, though in most of 

 them the amount of nephelite is so small and the transition so 

 gradual that, as a whole, they had best be called trachytic 

 phonolites. On this account Dannenberg did not distinguish 

 between the trachytes and the phonolites on his geological 

 map, though Deprat considers that Dannenberg has under- 

 rated the importance of the latter. My observations led me 

 to believe that the phonolites are of comparatively slight 

 importance, and they would appear to be, in general, schlieric 

 patches in the trachytic mass, distinguished by only very slight 

 differences in composition, rather than distinct flows, radically 

 different in character from the trachytes. 



Among prominent localities may be mentioned the summits 

 of Monte Oe and Monte Tinzosu near Santu Lussurgiu, where 

 they overlie trachyte but without discontinuity ; a small hill 

 near Cuglieri west of Castel Ferru, with fine columnar struc- 

 ture ; and a ridge west of Punta sa Chidanza, marked 872 on 

 the map (206, III). 



Description. — Megascopically these rocks are extremely fine- 

 grained and compact, looking much like the densest quartzites, 

 and often showing well-marked conchoidal fracture. The 

 color is dark gray, usually greenish or brownish, and often 

 with a slightly greasy or silky luster. They are almost abso- 

 lutely aphyric, the only phenocrysts present being very rare 

 feldspars, 5 to 10 mm in diameter. In some cases a flow texture 

 is well developed, this being especially fine at Monte Tinzosu. 



Am. Jour. Sci.— Fourth Series, Vol. XXXIX, No. 233— May, 1915. 

 34 



