wald, which is connected
with another volcanic district of Central Germany known as the
Vogelsgebirge. The highest point in the range is attained in the
Lohrberg, which rises 1355 feet above the sea; the next, the Great
Traenkeberg, 1330 feet; and the next, Great Oelberg, 1296 feet.

[Illustration: Fig. 21.--The Volcanic Range of the Siebengebirge, seen
from the left bank of the Rhine, above Bonn.--(Original.)]

The range consists mainly of trachytic rocks--namely,
trachyte-conglomerate, and solid trachyte, of which H. von Dechen makes
two varieties--that of the Drachenfels, and that of the Wolkenburg. But
associated with these highly-silicated varieties of lava--and generally,
if not always, of later date--are basaltic rocks which cap the hills of
Petersberg, Nonnenstrom, Gr. and Ll. Oelberg, Gr. Weilberg, and Ober
Dollendorfer Hardt. The question whether there is a transition from the
one variety of volcanic rock into the other, or whether each belongs to
a distinct and separate epoch of eruption, does not seem to be very
clearly determined. Mr. Leonard Horner states that it would be easy to
form a suite of specimens showing a gradation from a white trachyte to a
black basalt;[5] but we must recollect that when Mr. Horner wrote, the
microscopic examination of rocks by means of thin sections was not known
or practised, and an examination by this process might have proved that
this apparent transition is unreal. According to H. von Dechen, there
are sheets of basalt older than the greater mass of the brown coal
formation, and others newer than the trachyte;[6] while dykes of basalt
traversing the trachytic lavas are not uncommon.[7]

The trachyte-conglomerate--which seems to be associated with the upper
beds of the brown coal strata--is traversed by dykes of trachyte of
later date; and though it is difficult to trace the line between the two
varieties of this rock on the ground, Dr. von Rath has recognised the
general distinction between them, which consists in the greater
abundance of hornblende and mica in the trachyte of the Wolkenburg than
in that of the Drachenfels.

The trachyte of the Drachenfels was probably the neck of a volcano which
burst through the fundamental schists of the Devonian period. It is
remarkable for the large crystals of sanidine (glassy felspar) which it
contains, and has a rude columnar structure.

The absence of any clearly-defined craters of eruption, such as are to
be found in the Eif