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. In a 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. 



JVow for the Trap. — In 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, hke 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 

 tliis porous vesicular rock are easily lorn down wath the pick axe, and 

 carried away to mend the roads. In hand specimens, I could scarce- 

 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- 



■" How then do we know where the line of junction is ? By observing the gradu- 

 ated appearance both ways, and the perfectly distinct surfaces which, perhaps at a 

 fittle distance to the right or left, we may discover. 



