160 THE ROMAN COMAGMATIC REGION. 
especially true of the salfemanes and the more lenic dosalanes, among which the 
only hyaline types observed are small flows and the borders of larger ones at Mount 
Vesuvius. It is possible or probable that similarly vitreous portions existed origi- 
nally at the other centers, but have been removed by erosion. In any case the 
volume of these is always small. 
Glass appears in small amounts as the rocks increase in salic, and especially 
feldspathic, components, a little glass cement being found in some types of ciminose, 
vulsinose, phlegrose, etc. The most hyaline rocks belong to the subrangs phleg- 
rose (I. 5. i. 3) and harzose (II. 4. 3. 3), and some types of the former areper- 
hyaline obsidians. It is to be noted in these cases that the vitreous types of these 
two subrangs may and often do form massive flows and are not confined to small 
trickles of lava or outer borders of large flows. Probably connected with this 
tendency of these magmas to solidification before complete crystallization is the 
tendency of the harzose to form flow breccias and that of the phlegrose lavas of 
the Phlegrean Fields to form dustlike tuffs rather than solid flows.* 
This tendency of the Roman magmas to form holocrystalline rather than 
hyaline types is probably connected with their generally high content in potash 
and lime. This view is borne out by the observed facts stated above, since the 
types of lenic orders of dosalane and of salfemane, which are uniformly holocrys- 
talline, are relatively the richest in these two oxides, while the types of harzose 
and of phlegrose are relatively the poorest and are both sodipotassic. But further 
discussion of this topic is uncalled for, as it is rather a general one than character- 
istic of the region. 
The abundance of phenocrysts is very noticeable, though this is not surprising 
in view of the fact that the rocks are effusive flows. It is more interesting to observe 
that the salic minerals, especially soda-orthoclase and leucite, are much more often 
phenocrystic than the alferric minerals, phenocrysts of augite and of olivine sur- 
passing those of the salic minerals only exceptionally. The large size of the pheno- 
crysts of soda-orthoclase and leucite has already been commented on. This 
tendency of the salic minerals to development in large crystals, while the alferric 
ones are small, is also evident in the rarity of megaporphyritic types among the 
rocks belonging to the more femic divisions, such as braccianose and albanose. 
In these the grain is uniformly small and megascopic phenocrysts are very rare 
and never attain large dimensions. 
Space Relations. 
For the discussion of the space relations or the geographical distribution of 
the different magmas and rock types it will be best to collate and summarize the 
statements made in the preceding petrographical descriptions as regards the occur- 
rences in the several districts. These will be supplemented, so far as possible, 
by the descriptions of other petrographers, though in most cases it is difficult, if not 
* Cf. Deecke, Geol. Fiihrer durch Campanien, Berlin, 1901, p. 44. 
