t
is known to architects as bond.

But, so far, this is only like "dry walling," the mass wants cementing
together to make it solid. The cementing process happens in this way
in our rocks, which are almost purely silicious: Water containing a
minute quantity of carbonic acid in solution, which most rain-water
does, especially when it comes into contact with decaying vegetation,
has the power of dissolving silica to a slight extent. This is proved
in various ways, and is shown in the fact that all river water
contains more or less silica in solution.

The circulation of water through the sand deposit of which our rocks
are made dissolves part of the grains, and the silica taken up is
redeposited on others. I cannot explain the chemical reaction that
produces this deposition, but that it takes place in the rock during
some period of its history is certain. I exhibit a quartzite pebble
taken from the Triassic sandstone at Stanlow Point, which, as can be
easily seen, was at one time worn perfectly smooth by attrition and
long-continued wear, for the quartzite is very hard. Upon this worn
surface you will see spangles and facets which reflect the light, and
on closer inspection it will be evident that they are crystals of
quartz that have been deposited upon the surface of the worn pebble
after it became finally enclosed in the rock.

A microscopic examination of the granules of the rock itself will show
that many of them have had crystalline quartz deposited upon their
surfaces, and in some cases rounded grains have in this way become
almost perfect crystals.

An examination of the best sandstones for building purposes shows that
they possess more of these crystalline particles than the inferior
ones, and a good silicious sandstone shows its good quality by a fresh
fracture sparkling in the sun. In addition to these crystalline
deposits of silica I believe it exists also as a cement which binds
the particles together when in contact.

It certainly is, however, with this secondary silica that the original
sand has become a building stone, and the particles have become
interlaced and bound together. Thus, in building parlance, the grains
are the rubble of the wall, the currents the quarrymen, masons and
laborers, and the silicious infiltration the mortar.

And now, when I am on the subject, I may point out that this hard and
compact quartzite pebble was also once loose sand. The only difference
between the sandstone i