I. The Rocks of Monte Ferru. 



517 



phenocrysts of any kind, though a few specimens show very 

 rare (not more than 1/2 per cent) small (l/2 mm ) crystals of 

 black angite or still fewer feldspar phenocrysts. 



The microscopic characters are, in general, quite uniform. 

 The rock is composed of very small thin laths of alkali feld- 

 spar, mostly un twinned, with some small anhedra of a colorless 

 or pale greenish-gray diopside-augite, with extinction up to 

 40°, and fewer small magnetite grains, in a colorless, isotropic, 

 glassy base. The feldspar laths generally show fluidal arrange- 

 ment, but are occasionally divergent. X either quartz nor 

 andesiue could be detected in these trachytes, but a few showed 

 a very small amount of nephelite, revealed by staining, show- 

 ing them to be transitional to the phonolitic trachytes. Small 

 crystals of blue noselite are very rare, and these were not 

 found to be as abundant in my sections as the descriptions of 

 Doelter and Deprat would lead one to expect. Neither horn- 

 blende nor biotite are present, except for a few crystals of the 

 latter in the trachyte of Punta sa Chidanza. 



Chemical Composition. — The chemical composition of both 

 types may be discussed together. Two analyses made by me 

 of a typical specimen of each are given in Table I, along with 

 analyses of similar rocks for comparison. 



Table I. 



Analyses of Trachytes. 



A 



Si0 2 58-43 



A1 2 3 18-58 



Fe 2 3 3-00 



FeO 1-22 



MgO... 0-13 



CaO 3-50 



Na 2 .... 4-78 



K 2 5-82 



H 2 + ... 0'94 

 H 2 0-„. 1-63 



Ti0 2 1-11 



Zr0 2 0-24 



PA—- 0-19 



so 3 0-11 



MnO 0-09 



BaO 0-14 



SrO 0-06 



99-97 



B 

 61-07 

 19-47 

 2-50 

 0-71 

 0-46 

 145 

 4-84 

 7-07 

 0-96 

 0-42 

 0-69 



0-04 



trace 



99- 



C 

 57-01 

 20-81 

 4-13 

 n.d. 

 1-23 

 2-91 

 5-92 

 6-30 

 1-41 



n~d. 



trace 



trace 



D 

 55-11 

 20-90 

 6-11 

 n.d. 

 1-21 

 354 

 5-31 

 7-52 

 1-04 



n.d 



E 

 61-62 

 18-11 

 2-36 

 1-28 

 056 

 1-44 

 5-77 

 7-60 

 0-78 



0-87 



trace 0-13 



CI 0-15 



trace trace 



99-72 100-74 109-67 



F 

 60-20 

 20-50 

 1-58 

 1-01 

 0-39 

 1-96 

 4-75 

 5-57 

 3-00 



0-51 



n.d. 



n.d. 



99-47 



A. Trachyte (I". 5. 2. 3). Molino, near Cuglieri, Monte 

 Ferru. H. S. Washington analyst. 



