ITALIAN PKTROJ.OGICAL SKETCHES 5 5/ 



rouiulccl outlines, but those of a larger size offer more variety. 

 While some give sharp, normal, eight sided sections, with a few 

 small, isolated inclusions, the greater number show more or less 

 perfect skeletal forms. Their sections consist of simple four- 

 armed crosses ; eight-rayed stars with four alternate arms larger 

 than the intervening ones, and thick at the ends or with a short 

 process projecting from the end on each side ; six-rayed stars 

 with thick and equally developed arms at angles of 6o°; finally, 

 sections resembling equilateral, spherical triangles in outline, 

 generally with triangular spots of glass at the apices. 



The forms are evidently due to sections of skeleton crystals, 

 and all may be referred to leucite skeletons like those described 

 by Senigaglia' in a lava of 1753 from Vesuvius. They are 

 developed not so much along axes as along planes from the 

 center of the crystal to all the trapezohedral edges, the crystals 

 being really trapezohedrons with deeply sunken faces and high 

 salient edges. 



Similar forms are to be found in other Italian leucitites, as 

 from Lake Bracciano and the Alban Hills. Pirsson^ has 

 observed almost identical forms in rocks from the Bear Paw 

 Mountains in Montana and gives an interesting discussion of 

 their growth to which the reader is referred. 



Another typical leucitite comes from Sassi Lanciati (Tossed 

 Rocks), a locality about 2*^™ south of Bolsena on the lake shore 

 where the road passes a lava stream about 10 meters high, 

 showing well developed columnar structure. In the early days 

 of geology this was apparently a well-known and oft-cited local- 

 ity, but more accessible and better examples of this structure 

 have thrown it into oblivion. 3 



The rock is described by Klein (p. 21), and is very similar to 

 the first one described in this paper. It may be noted however 

 that the inclusions in the leucite are rings of augite microlites 

 arranged tangentially near the borders, and that no skeleton 



' Senigaglia, Neu. Jahrb., B. Bd. VII, 418, 1891. 

 *PiRSSON, Am. J. Sci., I, 1896. 

 3VoM Rath, op. cit., 292. 



