ITALIAN PETRO LOGICAL SKETCHES 84 1 



])act, and extremely tough, while those forming ejected blocks 

 in the tuff are so decomposed as to be very friable. 



The flow specimens show a number of flat vesicles whose 

 walls are smooth and occasionally coated with small crystals of 

 nepheline, as was noticed by vom Rath. The groundmass is 

 dark gray and very compact and aphanitic. 



Through this are scattered leucite crystals in profusion, 

 which make up in places over a third of the bulk of the rock 

 and seldom fall below one-quarter, giving the rock a most char- 

 acteristic appearance. The crystals are of good size, generally 

 0.5 to i.O'-"'" in diameter, usually perfectly formed with sharp 

 edges, but occasionally in fragments. While sometimes fresh 

 and of the usual gra}' color and waxy luster (in one case yellow- 

 ish through infiltration of ferruginous water), they are usually 

 dull white, due to alteration. In the most altered specimens 

 this kaolinization reduces them to a very friable and mealy con- 

 dition. Thev include grains of auo-ite and magnetite, which are 

 irregularly arranged or clustered at the center. Apart from the 

 leucites few phenocrysts are present, there being some sanidines 

 and a few black augites. 



In thin section the leucite-trachyte of Madonna di Lauro^ 

 near Vetralla, and that from a stream in the crater wall of Vico 

 below San Rocco, which are typical of my Vico leucite rocks, 

 present much the same characters. The large leucites are much 

 cracked and exert little action on polarized light, the character- 

 istic twinning being seen only in a few places. This is probably 

 due to incipient or, as in the Vico rock, to evident alteration. 

 There are not very abundant inclusions of augite, feldspar, mag- 

 netite, and glass. In the yellow leucites of one rock deposits 

 of limonite are noted along all the crevices, while the leucite 

 substance itself is quite colorless. 



Feldspar phenocrysts are not uncommon, those of alkali 

 feldspar being largely in the majority over those of plagioclase, 

 which is a basic labradorite. Light brown glass inclusions are 

 common, generally clustered toward the center. The pyroxene 



■ Thi.s is described by Bucca, op. cit.. 61. 



