I. The Rocks of Monte Ferru. 521 



A. Phonolite (I(II). 5(6). 1. 3). Monte Tinzosu, Monte Ferru. 

 H. S. Washington analyst. 



B. Phonolite (I". (5)6. 1. 3(4). Hill near Castello Ferru, 

 Monte Ferru. H. S. Washington analyst. 



C. Phonolite. Hill near Castello Ferru. C. Doelter analyst. 

 Denkschr. Ak. Wiss. Wien, xxxix, p. 62, 1878. 



D. Trachyte. Sennariolo, Monte Ferru. H. S. Washington 

 analyst. Cf. p. 515. 



The two analyses are very similar, except that the alkalies 

 differ slightly. Both of them are also much like the analysis 

 of the Sennariolo trachyte, though in these the soda is markedly 

 higher, this difference involving the modal content in nephelite. 



Doelter's analysis is much lower in silica and higher in soda. 

 He does not mention the exact locality where his specimen was 

 obtained, but it is probable that it comes from the same local- 

 ity as that analyzed by me — a small hill (592 on the map, 206, 

 IV), west of the castle and about one kilometer and a half 

 southeast of Cuglieri along the fourth class road. This shows 

 well-developed columnar structure, the lava resting on tuffs, 

 giving a fine exposure, and one from which a specimen would 

 certainly be taken by any geologist. If this is so the differ- 

 ences between Doelter's and my figures are hard to explain, as 

 the mass is apparently very uniform in character. It may be 

 noted that the differences here are much the same as those 

 between Doelter's and my analyses of the Sennariolo trachyte. 



The norms of the two rocks are as follow : 



A B 



Or 51-71 40*59 



Ab 25-94 37*20 



An _ __ 0-28 



Ne 10-65 11-36 



Ac _. 4-62 



Ns 0-24 



Di 3-35 2-81 



Wo 1-28 1-39 



Mt 1-39 



II 0-68 1-22 



Hm 096 



These place the Tinzosu rock (A) in phlegrose, transitional 

 to beemerose, I(II). 5(6). 1. 3, while the other, B, is in beeme- 

 rose, transitional to miaskose and phlegrose, with the symbol 

 Y . (5)6. 1. 3(4). They both indicate that the amount of nephe- 

 lite present is not great — a fact fully in accord with their close 

 similarity to the true, nephelite-free trachytes. It may also be 

 noted that the presence of normative acmite in A, while it is 

 absent in B, is in harmony with the fact that the pyroxene of 



