520 Washington — Contributions to Sardinian Petrography: 



On the whole, except for the greasy luster in some specimens, 

 they closely resemble the trachyte of the Sennariolo type. 



In thin section also these rocks are very similar to the Sen- 

 nariolo trachyte. Small laths of alkali feldspar make up most 

 of the rock, but these are usually rather larger' and not so well 

 defined as in the trachyte. There are many small ('02 — -10 mrn ) 

 prismoids of pale green segirite-augite, with the usual, though 

 faint, pleochroism. The presence of these rather readily dis- 

 tinguishes the two rocks, as in the trachyte the small pyroxenes 

 are almost colorless or only slightly greenish gray. Magnetite 

 grains are rare and there is no soda-lime feldspar. No noselite 

 was seen in my sections, though one analysis indicates that it is 

 present, and both Doelter and Deprat speak of it as a constant 

 constituent. Between the feldspar laths is a weakly birefringent 

 base of nephelite, which is not easily discernible and is best 

 revealed by staining. In a few specimens, as in that from the 

 hill below Castel Ferru, the nephelite forms very small, sharply 

 defined, equant individuals. All the phonolites seem to be 

 noncrystalline. 



Chemical Composition. — An analysis was made of the pho- 

 nolite of Monte Tinzosu, which was regarded as most typical, 

 and of one from the small hill west of that on which stands the 

 Castello Ferru mentioned above. The latter is incomplete, as 

 only the main constituents were determined for comparison 

 with Doelter's analysis. The amount of Zr0 2 is assumed to be 

 that in the other phonolite, the alumina being corrected for it. 



Table II. 

 Analyses of Phonolites. 



A B C D 



Si0 2 -.... 60-43 5947 53'95 61*07 



A1 2 3 18-35 1925 23-82 19-47 



Fe 2 0, ... -- 1-64 1-92 2-68 2'50 



FeO\_ _ 0-91 0-94 n.d. 0*71 



MgO 0*17 0-52 0-55 0-46 



CaO 1-41 1-66 0-99 1*45 



Na 2 6-15 6-86 10-03 4-84 



K 2 ._ _ 8-68 6-89 5*79 7*07 



H 2 + 0-62) 0-96 



H 2 0-__ 0-34 f 128 189 0-42 



Ti0 2 0-36 0-66 _. 0-69 



Zr0 2 0-21 (0-20) 



P,0 6 _-. trace .. _- 0'04 



S0 3 0-22 _. trace 



CI none .. ._ __ 



MnO._ _ 0-16 __ trace trace 



BaO 008 



SrO _ 0-02 



99-75 



99-65 



99-fO 99*68 



