846 HENRYS. WASHINGTON 



nepheline in the groundmass. The apparently total absence of 

 leucitites, which is commented on by Mercalli, is a most striking 

 fact, and in great contrast to the adjacent centers of Bolsena 

 and Bracciano. 



Phojiolites. — The presence of these rocks would seem to be an 

 exception to the statement on a previous page that the Viterbo 

 rocks can be divided into only two classes. They occupy, in fact, 

 an exceptional position, occurring only as blocks in the last tuff 

 ejected by Monte Vico and in a couple of dikes observed by 

 Deecke. Their total amount is moreover very small, quite sub- 

 ordinate to those of the ciminites and leucite-trachytes. Of 

 these rocks I collected three specimens. One is from a loose 

 block in the yellow tuff west of Viterbo, another from a block in 

 the tuff at the Posta on the north edge of the Vico crater. They 

 are very compact and tough, with no great tendency to break 

 into slabs under the hammer. In the aphanitic, greenish gray, 

 slightly greasy groundmass are a few small glassy sanidine phen- 

 ocrysts, stained light brown in places, still fewer small dark 

 pyroxenes, and here and there a bright blue speck of haiiyne. 

 The sp. gr. is 2.509 at 10°. 



Under the microscope it presents a normal phonolitic struc- 

 ture of Rosenbusch's nephelinitoid type. The large orthoclase 

 phenocrysts are clear and free from inclusions, though a little 

 brown limonitic material lies along the cracks. They are tabular 

 parallel to the clinopinacoid, show commonly Carlsbad twinning, 

 and often have corroded outlines. Mantles of orientated alkali 

 feldspar are common. The rather rare augite phenocrysts are 

 well formed and of an olive green color, and quite strongly 

 pleochroic. Since the axis of greatest elasticity a makes an 

 angle of about 30° with the vertical axis they belong to the 

 aegirine-augites. 



Rather large haiiynes are quite abundant and frequently show 

 octahedral and cubic planes. They are generally colorless in 

 the interior and bright blue toward the edge, though sometimes 

 blue all over, or brown inside and blue out. They show the 

 peculiar inclusions arranged in fine parallel straight lines, often 



