936 THE JOURNAL OF GEOLOGY. 
in amount to the basalts and andesites. As surficial flows they 
are basaltic in character, being dark colored and heavy, with 
olivine among the phenocrysts in most cases. They are massive 
and compact or vesicular. They are porphyritic in some cases, 
but not noticeably so in others. They generally exhibit a semi- 
waxy luster that suggests the presence of nepheline, which 
however is not present. The luster is due to the alkali-feldspars 
in the groundmass. They are often dull greenish black owing 
to the serpentinization of olivine. As dikes they are basaltic in 
some cases, and phonolitic to trachytic in others, being gray in 
various shades, and having a somewhat waxy luster, due to 
alkali-feldspars. They are porphyritic or not in different cases, 
and range from aphanitic to phanerocrystalline. 
They present quite a range of composition within limits, 
and form a series of varieties connected by gradual transitions. 
They could not be embraced by any one definition and must be 
divided into several classes. 
The chief characteristics of the most basic class are the pres- 
ence of abundant phenocrysts of olivine and augite, and the 
absence of phenocrysts of feldspar. The groundmass may be 
anything from a dark glass to an almost phanerocrystalline, 
light gray mass. It is oftener aphanitic and dark greenish gray. 
The phenocrysts are large and pronounced in many cases, but 
are quite small in others. Chemically they are low in silica, 
from 46 to 52 per cent.; low in alumina, from g to 12 per cent.; 
high in magnesia, from 8 to 13 per cent.; moderately high in 
alkalies, with potash higher than soda, except in one case. The 
molecular ratio of the alkalies to silica is .o8 and .og. (See 
Table I). After the crystallization of abundant phenocrysts of 
olivine and augite the remainder of the magma, owing to low 
alumina and relatively high alkalies was so constituted that 
alkali-feldspathic minerals might crystallize out, which they did 
or not according to the conditions under which solidification took 
place. 
The principal characteristics of the second class are the pres 
ence of phenocrysts of labradorite, augite and olivine, in a 
