Arkell.1956.Jurassic..

io6 WESTERN GERMANY
than ioo volcanic necks are filled with tuff and breccia. In some of these
blocks of limestone have fallen down hundreds of feet below their original
level.
Another and more characteristic feature of both the Swabian and
Franconian Alb is the prevalance of sponge-reefs through all parts of the
Oxfordian and Kimeridgian; that is, through a limestone series which
reaches a total of not less than 650 m. The presence of these reefs greatly
complicates the stratigraphy, for from place to place any part of the
FIG. 12.—Jurassic outcrops of the Swabian and Franconian Jura.
succession is liable to pass laterally into unfossiliferous, unbedded mudstone
or saccharoidal, subcrystalline limestone or dolomite, and to swell
out into lenticular or ramifying knoll reefs. Ammonite or other fossil
zones can seldom be traced through the knoll reefs and the normal bedded
succession is completely interrupted by them, or liable to arch over them
(Fischer, 1913; Dorn, 1932; Roll, 1934; the sponges monographed by
Kolb, 1910). In some places (as near Balingen) sponge reefs begin in the
Transversarium Zone and continue in force in the same general area
through all subsequent zones. Elsewhere sponge growth came and went
more fitfully, until in the Pseudomutabilis and Beckeri Zones it spread
through the entire length and breadth of the Alb: possibly as the result
of a retreat of the shore-line from the rising Black Forest massif and the
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