The system is also
used for breaking up detached blocks of stone into smaller
sizes.
Quarrymen have, ever since the introduction of blasting, tried to
direct the blast so as to save stock. Holes drilled by hand are seldom
round. The shape of the bit and their regular rotation while drilling
usually produce a hole of somewhat triangular section. It was
observed, many years ago, that when a blast was fired in a
hand-drilled hole the rock usually broke in three directions,
radiating from the points of the triangle in the hole. This led
quarrymen to look for a means by which the hole might be shaped in
accordance with a prescribed direction of cleavage.
The oldest sandstone quarries in America are those at Portland, Conn.
It was from these quarries that great quantities of brownstone were
shipped for buildings in New York. The typical "brownstone front" is
all built of Portland stone. As the Portland quarries were carried to
great depths the thickness of bed increased, as it usually does in
quarries.
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