MODE OF EXTRUSION 43 
hold on the) bottom of the river bed and sometimes small basalt 
cliffs on one or on both sides of the valley. 
The outbreaks are thus fissural ones, tuffs and volcanic agglo- 
merates are not quite wanting, but they are scarce and saved from 
erosion only on several points of smaller importance by reasons of 
‚conditions related below. It may be supposed strange, that basalt 
beds of such a mightiness may influence the sedimentary series to 
such a small degree, but even in center points of basalt develop- 
ment the bordering palaeozoic strata are by no means, if at all, 
dislocated in such a degree as would be expected by a single out- 
break of these dimensions: the strata on both sides of such a mighty 
„basalt stream“ correspond roughly each to the other stratigraphi- 
cally and in relative height. 
A travel through these countries along the principal rivers — the 
only ways of communication in summer time — crossing miles after 
miles central areas of basalt development leaves the impression of 
a uniform basait plateau, due to one or several concordant lava 
flows of important mightiness, without any eminent physiographical 
peculiarities; in retiring from the central areas the river erosion 
eats through the lava cap and the floor rock appears in undisturbed 
position; the lava bed thins out, but always crowns the upper edge 
of the valley and the valley broadens immediately. A cross section 
through such a valley with uniform basaltic slopes reveals behind 
the upper edge an ideal plateau of almost absolute horizontality, 
strewn with lichen-covered heavy rock-débris; the basalt persists 
only in the immediate vicinity of the upper edge and then sedi- 
. mentary rocks of the main palaeozoic type replaces it. The boun- 
dary line between basalt and palaeozoic limestone (marked by a 
different flora of lichens) runs roughly parallel with the main valley. 
In places, the basalt edge continues outward to a narrower or broader 
horizontal plateform, from which a gentle slope leads up to the true 
plateau. The mentioned boundary runs then at the base of this 
slope; the contact line, even here covered with rock-débris, is more 
