portion of which was all-rock section. It was at that point that
openings were first used extensively and methods were developed, which
would not have been possible except where ears could be passed through
the shield. The bottom-heading method was first tried, but the working
space in front of the shield was cramped, and but few men could be
employed in loading the cars. To give more room, the heading was
gradually widened. The enlargement at the top, when made from the
shield, blocked all work at the face of the heading while the former
operation was in progress. To reduce the delays, the heading was raised,
thus reducing the quantity of rock left in the top, and the bottom was
taken out as a bench. To avoid blocking the tracks when firing the top,
a heavy timber platform was built out from the floors of the middle
working compartments. Most of the muck from the top was caught on the
platform and dropped into cars below. This method of working is shown by
Fig. 2, Plate LXVII. The platforms were not entirely satisfactory, and,
later, the drills in the heading were turned upward and a top bench was
also drilled and fired, as shown by Fig. 3, Plate LXVII. There was then
so little excavation left in the top that the muck was allowed to fall
on the tracks and was quickly cleared away. The method just outlined is
called the center-heading method, and was the most satisfactory plan
devised for full-rock sections.

_Excavation in Part Rock and Part Earth._--This was probably the most
difficult work encountered, particularly when the rock was covered with
boulders and coarse sharp sand which permitted a free escape of air. It
was necessary, before removing the rock immediately under the soft
ground, to excavate the earth in advance of the shield to a point beyond
where the rock was to be disturbed, and to support, in some way, the
roof, sides, and face of the opening thus made. The hoods were designed
mainly for the purpose of supporting the roof and the sides. With the