42 HENRY S. WASHINGTON 



The groundmass is almost holocrystalline, and is made up 

 chiefly of small round leucites. In two cases these contain only 

 few peripheral inclusions of augite and apatite needles, a peculiar 

 feature being that around the leucite crystals proper as defined 

 by these rings, is a late growth of leucite which extends irregu- 

 larly and acts as a mesostasis for the other constituents. In 

 rock from the second locality mentioned the inclusions are more 

 numerous, and in spots due to skeleton growth, as has been 

 previously described, though the skeleton forms are not as per- 

 fect as some of those seen elsewhere. 



Between these leucites is an interstitial mass of green or 

 greenish brown augite needles and grains, with some magnetite 

 grains, with the latter being associated flakes of orange red 

 hematite. Spots of a colorless feebly doubly refracting substance, 

 giving bright grays of the first order in m}- ver}^ thin sections, are 

 referred to alkali feldspar, as treatment with acid revealed no 

 nepheline. No melilite was seen. Glass base is present in a 

 very small amount. An analysis of one of these leucitites is 

 given in Table I, No. 5. 



The leucitite of Santa Maria di Galera,' which belongs to this 

 volcano, resembles the above very closely under the microscope, 

 though it is much coarser in structure. A rock very similar to 

 these is found as loose blocks in the yellow tuff of Monte Vir- 

 ginio. This is very much finer grained than any of the preced- 

 ing ; contains some plagioclase but no glass. 



Leucite -te phrite . — This seems to be quite common in the 

 region though not so much so as leucitite. It is represented by 

 a specimen from a lava stream in the crater wall just below the 

 town of Bracciano. It is rather dark and fine grained, but rough 

 in texture and not aphanitic. Scattered through the groundmass 

 are very many small clear leucites and some very small black 

 augites. 



Under the microscope it closely resembles the similar rock 

 of the Bolsena region, having the same doleritic structure. The 

 irregularly shaped leucites show strong double refraction, and 



' RosENBUSCH, Mikr. Phys. II, p. 1233, 1896. 



