BAVARIAN BASALTS 
113 
On the contact itself the sandstones are fritted, and the shales 
and the clayey sandstones are altered to jasper. A few feet away 
from the contact the sedimentary rocks are scarcely at all meta¬ 
morphosed; the original red coloration alone is perhaps a trifle 
paler; but at a distance of even three or four feet no change is 
observable. Finally, in some localities are found real lava out¬ 
flows, but these are seldom of any considerable extent. 
The structure of the basalt masses is generally distinctly colum¬ 
nar. Sometimes the columns are very heavy and measure four 
feet or more in diameter; but in other instances they are of slight 
proportions and small diameter. Often there are noticeable hori¬ 
zontal partings. Among the dikes the columns are as usual per¬ 
pendicular to the side-walls. 
Macroscopically, the different rock occurrences are hardly dis¬ 
tinguishable from one another. All are black, heavy and dense, 
with splintery fracture. Generally neither the phenocrysts of 
olivine nor augite are apparent; but occasionally they are dis- 
cernable in some of the small dikes. The most conspicuous char¬ 
acteristic of the basalts is the inclusions of peridotitic fragments 
(olivine fels) which are everywhere in evidence. 
In thin slices under the microscope three distinct types of basalt 
are readily distinguishable: (1) Nephelite basalt, (2) feldspar 
basalt, and (3) nephelitic feldspar basalt. 
In the nephelite basalts occur well bounded phenocrysts of oli¬ 
vine, and brown titanium augite, the latter having a violet tint. 
This augite displays zonal structure, especially the hour-glass sort. 
It exhibits strong dispersion, mostly in sections cut perpendicular 
to the optic axis B. Of the nephelite basalts two sub-varieties are 
to be distinguished, (a) Those in which phenocrysts of olivine 
prevail, the augite is quite rare. The ground-mass consists of 
abundant microlites of brown augite and magnetite*. Nephelite 
is present, but in fragments closely clustered but without crystal 
form. In the groundmass biotite is also found, usually in the 
nephelite groups. Glass is lacking, (b) In the sub-type in which 
the phenocrysts of olivine and titanium augite are present in 
equal proportions the ground-mass is composed also of augite and 
magnetite microlites, the interstices of which are filled with form¬ 
less nephelite and sometimes with brownish glass. Biotite is not 
present. 
