234 Dr. Cooper on Volcanoes 
Reply. So does the Lava of tna over shell Limestone ; 
so does the Lava of Dominica, St. Kitts, &c. the Lava cov- 
ers the alluvial of the shores of the Tibur near Rome. 
Further; if Lava and Basalt rocks, are dissociated with, 
and unconformable to other rocks—following no law of su- 
perposition or alternation, as the case is—then there is no 
reason why they should not cover and alternate with rocks 
of comparatively recent formation. 
Basa t and trap rocks afford aqueous vapour on dis- 
tillation : Lavas do not. 
eply. The newest fleetz trap formation is much older 
than any modern Lava: and therefore has undergone more 
exposure to the causes of disintegration. | 
the porphyries, it is probable they were of the formation 
now disputed. 
The only instance in which I have observed a figurate form 
(tetrahedral) that can create any doubt on the subject, is in 
some primitive traps within two or three miles of Philadel- 
phia. But the jointed, articulated prisms and columns of — 
etz trap basalt, are no where else found but among de- 
cided Lavas. 
(d) Streams of Lava are comparatively narrow at their 
source, and extend in breadth as they flow; they vary 1 
thickness ; they are never in very thin, or parallel and hor!- 
zontal layers. Basalts of the ficetz trap formation, on the 
contrary are so ; and affect an equable thickness in the 
same, and in different layers. ; 
Reply. All this depends on the degree of fusion. If the 
heat has been great and the stones fusible, the lava-stream 
in thin fusion, will extend in breadth as it flows. If other- 
wise, it will not. Basalts of all kinds and descriptions 0 
this respect, are so fully described by Soulavie, that his ac- 
