234. Dr. Cooper on Volcanoes 
Reply. So does the Lava of Atna 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. 
(6) Basalt and trap rocks afford aqueous vapour on dis- 
tillation : Lavas do not. 
Reply. The newest fleetz trap formation is. much older 
than any modern Lava: and therefore has undergone more 
' exposure to the causes of disintegration. 
Also, it is obvious that every rock that admits of infiltra- 
tion, and where pores are filled with crytals apparently for- 
eign to the rock, must contain moisture. This is the ease 
with porous and amygdaloid Lavas, as well as flcetz basalt. 
(c) The prismatic figurate form, is common to many por- 
hyries and shists ; to gyps, and to compact Limestone, as 
seein observed in the Pyrenees. 
Reply. This form may be found accidentally and rarely 
in the shists, limestones and gypsums mentioned. It is 
not a common but avery uncommon occurrence. As to 
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 
fleetz trap basalt, are no where else found but among de- 
cided Lavas. 
(d) Streams of Lava are comparatively narrow at their 
source, aud extend in breadth as they flow ; they vary in 
thickness ; they are never in very thin, or parallel and hori- 
zontal layers. Basalts of the floetz 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 in 
this respect, are so fully described by Soulavie, that his ac- 
