1. The Rocks of Monte Ferru. 515 



covered with a thick mantle of basalt, which extends far 

 beyond the boundaries of the trachytic mass, and a few very 

 subordinate, final and aberrant types. In its general structure, 

 therefore, it much resembles the neighboring Monte Arci, 

 though here the salic core is largely of rhyolite.* There are 

 at the present day no hot springs or solfataric activity, so that 

 the volcano may be regarded as definitely extinct. 



Trachyte (Pulaskose, I. 5. 2. 3). 



Cuglieri type. — Though the trachytes of Monte Ferru are 

 rather uniform, yet.it will be convenient to refer them to two 

 types, which differ notably from each other. The first is 

 markedly porphyritic and, at least in part, earlier than the 

 second type, and much less abundant. It is the sanidine- 

 plagioclase trachyte of Doelter, and the porphyritic type of 

 Deprat, while Dannenberg does not seem to discriminate 

 between the two. I found it well developed below Cuglieri, 

 specially at a mill (molino) 3 km. from the town along the 

 road, at the second small round area of trachyte on Dannen- 

 berg's map. It also occurs near the Serbatoio, on the road 

 from Cuglieri to Santu Lussurgiu, and at a few other localities. 

 Doelter describes it from near the iron mines in the southwest- 

 ern part of the volcano, a locality which I did not visit. 



Megascopically it is dopatic with many stout tables or 

 equant individuals of feldspar, and some black augites, which 

 lie in a pure light gray, compact, aphanitic groundmass. Micro- 

 scopically it is holoerystalline. The feldspar phenocrysts are 

 mostly of soda-orthoclase, with fewer of oligoclase-andesine, 

 and the subhedral augite phenocrysts are brownish-green and 

 somewhat corroded on the borders. The highly predominant 

 feldspars of the groundmass are stoutly prismatic or almost 

 equant and do not show fluidal arrangement. The greater 

 part is soda-orthoclase, quite commonly in Carlsbad twins, but 

 there is considerable andesine. No quartz is - visible. The 

 augites of the groundmass are small, greenish-gray anhedra, 

 and there are some magnetite grains. No biotite or horn- 

 blende was seen. 



Sennariolo type. — This type is by far the most abundant 

 and it forms the greater part of the trachytic dome. It is 

 usually massive, but in places columnar and somewhat platy 

 structures are found. No vesicular forms were observed. 

 Megascopically it is extremely dense, compact, and tine-grained, 

 with a dull luster and often subconchoidal fracture. It is 

 generally a pure gray, varying from rather dark to almost 

 white. The type is almost absolutely perpatic — quite without 

 * Cf . H. S. Washington, this Journal, xxxvi, p. 577, 1913. 



