GEOGNOSY AND GEOLOGY. 207 
are known in tne transition slate, and some are known more recent than 
the chalk and the newest tertiary formations. In Auvergne they traverse 
granite, and in other countries phonolite and basalt, with which they not 
rarely exhibit unmistakable indications of a common origin and time of 
occurrence. 
B. Phonolite (Clinkstone}. 
This is intermediate between trachyte and basalt, and is, in most cases, 
presented as clinkstone-porphyry. It forms a great part of the Rhone 
mountain, where it is exhibited in forms similar to those of basalt, namely, 
in spherical masses, or in caps extending over other rocks. Its cleavages 
are generally flat and tabular, although columns likewise occur. The 
principal accompanying mineral substances are zeolites. 
Phonolite forms greater or less masses in Hohgau, in Bohemia, in the 
Rhoéngebirge (in Thuringia), in the Siebengebirge, and in France 
C. Basalt. 
This embraces true basalt, basalt amygdaloid, anamesite, dolerite, and 
basalt conglomerate. which, as modifications of one and the same rock, pass 
insensibly one into the other. Basalt occupies the most important placc 
among the volcanoid rocks; its mountains may not be so high as those o: 
trachyte, but are much more extended. It also constitutes veins anc 
penetrations between strata of stratified masses. _Dome-shaped mountains 
or hills are of frequent occurrence, as also those which are truncated or 
actually conical. Basalt veins are of various thickness, and are frequently 
in such connexion with the caps, as to render it satisfactorily evident that 
the vein is simply the pipe or space through which the molten mass has 
been injected to cover the superior strata. 
Basalts exhibit picturesque rock formation, especially when brought in 
contact with water, or where earlier cataclysms have given rise to valleys. 
This is the case on the Island of Staffa (pl. 52, fig. 6), on the Island of 
Tahiti (pl. 51, fig, 6), and in other places. 
Basalt oceurs under the most diversified forms, representing, in this 
respect, nearly all the rocks of which mention has already been made. 
Thus we may see the most beautifully regular columns of various lengths 
and diameter; globular and spheroidal formations are often combined with 
eocentric shelly cleavage. 
Basaltic amygdaloid is found on the exterior of the rocks in caps or veins, 
where bubbles of gas may distend the melted matter, leaving cavities on the 
cooling of the mass. These cavities subsequently became filled with the 
most beautiful crystallization. Small veins not unfrequently consist entirely 
of amygdaloid. 
Basaltic masses are found as well on the bottom series as upon the newest 
tertiary ; they traverse older plutonic, or other volcanoid rocks, as also ore 
637 
