126 Igneous Origin of some Trap Rocks. 
suspect it to be the same rock which it is below, and might even sup- 
pose that it was not, did we not trace the change by an almost imper- 
ceptable gradation. But this is not all. At the depth of about two 
feet, rather less than more, the altered sandstone begins to grow ve- 
sicular. Fine pin-hole cavities make their appearance ; they are very 
numerous, and the solid substance which surrounds them becomes 
semi-vitreous, and loses the appearance of sedimentary or fragmentary: 
matter ; as we ascend towards the trap, the vesicles increase rapidly 
in size, and at, and near the junction, they are both numerous and 
large. Ina word then, for two or three feet below the junction, the 
sandstone is greatly indurated and inflated, and these appearances 
are the most remarkable at the junction. 
Now for the Trap—tIn many places, it is so blended with the 
sandstone, that for a few inches on each side of the line of junction 
we can scarcely tell which rock is which ;* they look as if melted to- 
gether. But in some places the sandstone has been removed from 
below the trap, so that the latter projects overhead, like the roof of a 
portico, and we can look upon its under surface. There it is most 
remarkably inflated ; the cavities would often contain peas, or small 
bullets, and they are many times, so numerous, that the remains of 
the rock serve little more than to connect them, and vast quantities of 
this porous vesicular rock are easily torn down with the pick axe, and 
carried away to mend the roads. In hand specimens, I could scaree- 
ly tell it from the vesicular lava of Hecla. The vesicular character 
of this trap rock is most conspicuous at the junction with the sand- 
stone, and continues to be very distinct for two or three feet above ; 
but it becomes usually less conspicuous between three and four feet 
above, and this character rarely appears much, above four and five 
feet ; beyond which limit the rock commonly resumes its compact 
and firm structure, and its sub-crystalline appearance, which often 
vanishes entirely, or is greatly obscured where the vesicles are the 
most numerous. 
It has been already mentioned, that this trap is sometimes amor- 
phous, and at other times indistinctly columnar; between the rude 
columns, or between the masses which are contiguous, and have 
fissures between them, there is occasionally the same vesicular ap- 
oe How then do we know where the line of junctionis? By observing the gradu- 
ated appearance both ways, and the perfectly distinct surfaces which, perhaps at 2 
Jittle distance to the right or left, we may discover. ; 
