512 
ELEMENTS OF GEOLOGY. 
stream of lava, 
from 30 to 40 feet 
thick, which has 
proceeded from 
one of several 
volcanic craters, 
still extant, on 
the neighboring 
heights. 
In some masses 
of decomposing 
greenstone, ba¬ 
salt, and other 
trap rocks, the 
Basaltic pillars of the Kasegrotte, Bertrich-Baclen, half-way StlUv-t- 
between Treves and Coblentz. Height of grotto, from T ure IS SO COll- 
‘ ^ st)icuous that the 
rock has the appearance of a heap of large cannon balls. 
According to M. Delesse, the centre of each spheroid has 
been a centre of crystallization, around which the different 
minerals of the rock arranged them¬ 
selves symmetrically during the 
process of cooling. But it was 
also, he says, a centre of contrac¬ 
tion, produced by the same cool¬ 
ing, the globular form, therefore, 
of such spheroids being the com¬ 
bined result of crystallization and 
contraction.^ 
Mr. Scrope gives as an illustra¬ 
tion of this structure a resinous 
trachyte or pitchstone-porphyry in 
one of the Ponza islands, which 
rise from the Mediterranean, off the 
coast of Terracina and Gaeta. The 
globes vary from a few inches to 
three feet in diameter, and are of an 
ellipsoidal form (see Fig. 591). The 
whole rock is in a state of decom¬ 
position, “ and when the balls,” says 
Fig. 591. 
Chiaja di 
(Scrope.) 
Mr. Scrope, “ have been exposed a Giobiform pitchstooe. 
short time to the weather, they L^na, Me of Ponza.^ 
scale off at a touch into numerous concentric coats, like those 
of a bulbous root, inclosing a compact nucleus. The laminaa 
* Delesse, sur les Koches Globuleiises, Mem. de la Soc. Geol. de France, 
2 ser., tom. iv. 
