that, from their comparative softness, have
a nearly similar effect with the impalpable green earth in roughing the
surface under the burnisher.

We find figured by M'Culloch, in his "Western Islands," the internal
cavity of a pebble of Scuir More, which he picked up on the beach below,
and which had been formed evidently within one of the larger vesicles of
the amygdaloid. He describes it as curiously illustrative of a various
chemistry; the outer crust is composed of a pale-zoned agate, inclosing
a cavity, from the upper side of which there depends a group of
chalcedonic stalactites, some of them, as in ancient spar caves,
reaching to the floor; and bearing on its under side a large crystal of
carbonate of lime, that the longer stalactites pass through. In the
vesicle in which this hollow pebble was formed three consecutive
processes must have gone on. First, a process of infiltration coated the
interior all around with layer after layer, now of one mineral
substance, now of another, as a plasterer coats over the sides and
ceiling of a room with successive layers of lime, putty, and stucco; and
had this process gone on, the whole cell would have been filled with a
pale-zoned agate. But it ceased, and a new process began. A chalcedonic
infiltration gradually entered from above; and, instead of coating over
the walls, roof, and floor, it hardened into a group of spear-like
stalactites, that lengthened by slow degrees, till some of them had
traversed the entire cavity from top to bottom. And then this second
process ceased like the first, and a third commenced. An infiltration
of lime took place; and the minute calcareous molecules, under the
influence of the law of crystallization, built themselves up on the
floor into a large smooth-sided rhomb, resembling a closed sarcophagus
resting in the middle of some Egyptian cemetery. And then, the limestone
crystal completed, there ensued no after change. As shown by some other
specimens, however, there was a yet farther process: a pure quartzose
deposition took place, that coated not a few of the calcareous rhombs
with sprigs of rock-crystal. I found in the Scuir More several cellular
agates in which similar processes had gone on,--none of them quite so
fine, however, as the one figured by M'Culloch; but there seemed no lack
of evidence regarding the strange and multifarious chemistry that had
been carried on in the vesicular cavities of this mountain, as in the
retorts of som