36 Professor Sepawick on Trap Dykes 
the sandstone is much more compact, and breaks into amor- 
phous fragments. 
It must however be allowed that in some other localities the 
sandstone did not, under similar circumstances, appear to have 
undergone any modification. 
Perhaps, as a general rule, none of the changes above de- 
scribed are well exhibited, where the portion of the dyke, in 
contact with the horizontal beds, assumes the appearance of a 
wacké. Should this observation be sufticiently verified, it would 
seem to indicate, that the earthy texture of the dyke is, in some 
cases, rather due to its original mode of aggregation, than to 
any subsequent decomposition. I may, however, assert unequi- 
vocally, that I never saw any beds which are easily susceptible 
of modification (such as coal or carbonaceous shale) in immediate 
contact with the trap, without haying undergone a remarkable 
change. 
The overlying trap at Bolam bears no resemblance to a sub- 
stance which has been tranquilly deposited on the inferior strata ; 
for it is separated from them by a broken indented superficies 
which has exposed many distinct beds to its immediate action. 
Some of the massy columns rest on a bed of shale partially 
converted into a substance resembling Lydian stone, which 
rings under the hammer, or flies in all directions into a number 
of sharp splinters. Others are supported by a bed of impure 
coal or carbonaceous shale, in the upper part of which are found 
shapeless masses in various states of induration, mixed irregu- 
larly with angular pieces ef trap, and an earthy substance like 
soot or pounded charcoal. Where the carbonaceous ingredients 
are most abundant, the parts of the bed in immediate contact 
assume the appearance of coke derived from the artificial dis- 
tillation of impure coal, and not unfrequently separate into a 
number of minute prisms*. An impure carbonaceous powder is 
* See the Note to p. 33. 
