Mineralogy and Petrography. 47 
plagiocla-e-basalt, and nepheline-dolerite. The latter rock has 
been classed by Rosenbusch^ among the nephelinites because of the 
supposed non-existence of olivine in it The nepheline rocks have 
been erupted since the beginning of Tertiary time and are older than 
the plagioclase-basalt, which occurs in them in the form of dykes. 
The normal constituents of the nepheline rocks are augite, olivine, 
nepheline, apatite, biotite, and magnetite. The dolerite contains 
these as idiomorphic crystals in a groundmass compo-ed of micro- 
lites of the same minerals and plagioclase in a devitrified base. In 
the finer grained variety the nepheline occurs principally as the 
interstitial substance between the other constituents. In both 
varietie 
phillipsite, 
that its true nature can be distinguished only with great difficulty. 
Twins of this mineral parallel to P^ are not rare. Apatite is 
abundant, and frequently contains inclusions of the groundmass. 
Rubellan was discovered in a large number of sections, and hyalite 
and aragonite were found filling druse cavities. Both varieties of 
the nepheline rock are regarded as portions of the same magma. 
The dolerite is over the basalt, and is supposed to have cooled first. 
Inclusions of it are common in the underlying rock. Foreign in- 
clusions, found also in this rock, consist of augite and sanidine, of 
which the former is usually on the exterior. Other common con- 
stituents of these inclusions are hematite, green spinel, and orange- 
colored rutile. The plagioclase-basalt contains quartz inclusions 
surrounded by rims of augite crystals.— Prof. Judd^ calls attention 
to the fact that petrograpliical classification is based on the qualita- 
tive and not the quantitative determination of the constituents of 
rock masses. He shows that rocks composed of the same minerals 
may have widely varying compositions, even when their groundmass 
is approximately the same. Five examples of hypersthene andesites 
liaving the same mineralogical composition are taken, and it is 
shown that their content of silica ranges from 51. 8r^' to 705^. The 
fact that the same minerals are found in rocks possessing such dif- 
ferences in composition is explained by supposing them to have 
crystallized in the earlier stages of the rock's solidification and then 
to have been separated from the residual magma, and finally to 
have recombined with this in proportions different from those in 
which they first occurred. Since the residual portion is much more 
acid than the individualized portion, it is easy to imagine rocks of 
any degree of acifHty to have been formed by the mingling of the 
two portions in different amounts. The effect of the presence of 
water in lowering the fusing point of a rock is also discussed, in 
relation to its bearing on volcanic phenomena. 
MiXERALOGicAL News —Note.—ln the mineralogical notes for 
the current year the crystallographic axes will always be repre- 
