BAVARIAN BASALTS 
115 
detached by the basalt from the older rocks lying near the sur¬ 
face. In the north, these are granite; but in the south they are 
Triassic sandstones and shales (Keuper). 
The influence of molten basalt upon the granite is varied; in 
some cases the granite is easily recognized, while in others it is 
almost impossible to identify. In the first instance large frag¬ 
ments of rocks are at once recognized as granite. However, near 
the included mass the basalt contains well bounded diopside and 
becomes rich in a brown glass, which penetrates the granite. A 
little farther away from the basalt, the glass is colorless, and 
shows fluidal structure in and around the minerals of the granite. 
In this glass are found spinel, cordierite, and sillimanite. In the 
original orthoclase, or microcline, the perthitic structure is well 
preserved; but, the axial angle is very small, as in sanidin. The 
plagioclase (andesine) is mostly broken into small fragments, 
and these appear as if swimming, as it were, in the glass. The 
orthoclase is veined with the same glass. The quartz is partly 
altered into glass and contains abundant glassy inclusions. In¬ 
stead of biotite occurs a brown glass with countless octohedrons 
of spinel. 
In the second case in which the granite inclusions are scarcely 
recognizable macroscopically, the microscope at once proves the 
identity of them. The axial angle of the monoclinic feldspar in 
these granites is likewise very small, as in sanidine. The plagio¬ 
clase is nearly always surrounded and enclosed by a feldspar, 
showing the optical marks of sanidine. The same mineral is found 
in the cracks of the plagioclase. The quartz is broken by the heat 
and the cracks are filled with glass. In other instances the mineral 
is changed into chalcedony. Of the original mica, nothing is 
visible; in its stead, and at its expense, sillimanite, spinel, rutile, 
and anatase are formed. Cordierite is missing. 
In the immediate vicinity of the inclusions the basalt itself is 
also found to be altered. A dark zone encircles the granite frag¬ 
ments. The peculiarity of this zone is that, with an average 
width of only one to two mm, it contains much more magnetite and 
biotite than the ordinary basalt. Even the olivine is more thor¬ 
oughly decomposed than usual. This zone is followed by another 
zone, of about the same width, in which olivine is very rare, 
microlites of augite, magnetite, and biotite being the only con- 
