844 HENRY S. WASHINGTON 



considered.' In all the occurrences which I saw and in all my hand- 

 specimens the perfection of form of the tetragonal trisoctahedron 

 is very striking, and furthermore the abundance of the leucites 

 is too great to countenance the idea of their accidental presence. 

 The state of alteration does not seem to me to have any bearing 

 on the case, since this is dependent on surface conditions and 

 would go on the same no matter what the origin of the crystals. 

 The argument may rest on the ground that a trachytic magma 

 would be at a higher temperature than a leucitic magma, but 

 this idea is not expressed by either of the above writers. Vom 

 Rath's analysis of petrisco is given in Table II (No. 6), and 

 while it differs considerably from those of similar rocks from the 

 Bolsena and other regions in containing less lime and magnesia 

 and more silica and alkalis, yet it can be closely paralleled 

 by analyses of similar rocks containing undoubtedly primary 

 leucites to be described later. 



The rock composing the dome of Monte Venere, which forms 

 the last eruption of the region, is also a leucite-trachyte, but of 

 a somewhat different type. The groundmass is very compact 

 with only a few small vesicles, and the color is light gray. It is 

 very tough under the hammer — a characteristic of most of the 

 leucite rocks of all these regions. The leucite phenocrysts are 

 neither as numerous nor do they stand out as prominently as in 

 the preceding types, being small, quite fresh and glassy, and of 

 a pale grayish white color. There is an abundance of very small 

 phenocrysts of dark pyroxene and some small scales of biotite. 

 The sp. gr. of this rock is 2.609 ^^ 10° C. 



Under the microscope leucite is seen to be a much more 

 generally distributed constituent than in the preceding, or than 

 the megascopical examination would lead us to suppose. It runs 

 down from the larger phenocrysts to very small crystals of the 

 groundmass, of which it forms a very large part. It seldom 

 shows definite crystal boundaries ; and inclusions, as well as 

 double refraction, are rare. 



Feldspar phenocrysts are common, and plagioclase is more 



'Cf. RosENBUSCH: Mikr. Phys., I, 311, 1892, and II, 826, 1896. 



