PETROGRAPHY. 1 21 
Olivine. Phenocrysts: none to 2 per cent, o. 5 to 2 . o mm., subhedral to anhedral, equant 
to fusiform, colorless. Groundmass: about 4 per cent, o. 2 to 0.02 mm., anhedral, equant. 
Magnetite. Groundmass: about 2 per cent, 0.02 to 0.05 mm., anhedral, equant. 
Apatite. Groundmass: about 2 per cent, 0.05 to i.omm., subhedral, prismatic. 
Chemical composition and norm as on p. 119. 
Type specimen from flow of 1631, La Scala, near Torre del Greco, Mount Vesuvius, 
Campanian District. 
II. 8. 2. 2. Roma! Vesuvose [Leucitite, Rome Type]. 
This type is as yet unestablished by an analysis, so that a brief description of 
it must suffice. It is based on several specimens which, on careful comparison with 
those of similar types of undoubted braccianose and albanose rocks, were judged 
to be intermediate between them, differing from the former in the total absence of 
feldspar between the leucite and small augite crystals, nephelite or glass in small 
amount replacing it, and from the latter by the considerably greater amount of 
leucite as compared with augite, indicating a position in dosalane rather than in 
salfemane. These measurements could not be carried out very exactly, for reasons 
given above, but the differences are clearly seen on comparison of the sections. It 
may also be added that the occurrence of such a type is to be expected in the region. 
Megascopic characters. The rocks are very dark gray or almost black, dense 
and compact, showing very few phenocrysts of leucite and augite. The groundmass 
is aphanitic, or extremely fine-grained. 
Microscopic characters. These correspond in every way with those described 
later under romal albanose, except that leucite is slightly more abundant. The 
fabric is in most cases a clathrate one, composed of numerous small, round anhedra 
of leucite, with some pale greenish-gray augite microphenocrysts, and an interstitial 
microgroundmass of small felted augite prisms, often tangential toward the leucites, 
with many minute magnetite grains. No feldspar is present, but between the augite 
prismoids is a colorless cement which is either nephelite or glass. Olivine does not 
often occur in this type, but a minimal amount may be present in some cases. 
Chemical composition and mode. As no chemical analysis of this type was avail- 
able, the attempt was made to calculate the chemical composition from the measured 
mode. For this a specimen was chosen from a quarry opposite the railroad station 
of Oriolo, below Monte Raschio, in the Sabatinian District, as the amount of leu- 
cite seemed to be decidedly greater than in similar albanose rocks, and the grain was 
sufficiently large to allow fairly accurate measurements and with probably not very 
serious error from overlapping of the dark minerals. The results of this are seen 
below, in which the "augite" includes the small amount of nephelite base, the 
exact quantity of which is impossible to measure. 
Units Vol. %. Sp.gr. Wt. %. 
Leucite 1,041 =51. 0X2. 5 = 127.5 43 -8 
Augite 932 = 45.7 X 3.2* = 146.2 50.2 
Magnetite 68 = 3.3 X 5.2 = 17.2 6.0 
2,041 100.0 290.9 100.0 
*3_3 corrected for 12 per cent of nephelite. 
