Surface blasts were also tried upon this rock at the outset, in hopes that, by being placed in the most favourable positions between the sharp ridges of the rock, they might break them down. These, however, like those upon Tower Rock, entirely failed to produce any noticeable effect, even when they contained four and five hundred pounds of the best sporting powder. The drilling machine was therefore called into requisition as before, and used continuously till the completion of the work.
On account of the extent of this rock, a different plan of operations for its removal was adopted. One side of the rock most favourable for blasting was selected, and a row of holes drilled parallel to the edge, and at a distance from it equal to the depth of the holes, which was taken to extend 1 foot below the required level, 23 feet at mean low tide. After blasting out these holes, a new line of holes was drilled parallel to the former line, or to the “face” left by the blasts, and these also were blasted out; then a third line, and so on, progressing regularly across the rock, continually blasting it off in parallel blocks, extending downward a little below the depth required.
The advantages of this mode of operation were that it enabled the blasts to act laterally, in which direction they were the most powerful; and the rock was left, after each series of blasts, with a nearly vertical side, or “face,” in which the presence of seams could be more readily detected, and the character of the strata observed, so that the most favourable positions could be selected for the next blasts.
Sometimes the craters, following the strata, ran under, or left an overhanging “face,” in which case a large charge placed under its projecting edge, usually had the effect of throwing off the overhanging portion, and sometimes of dislodging large masses.
After the rock had been in this way blasted entirely across, and to the general depth required, a careful survey was made, the soundings being taken in lines from 5 to 10 feet apart, and at right angles to each other, the lower end of the sounding pole being placed by the diver alternately upon the highest and the lowest points.
This survey showed that although more than the required depth had been generally attained, yet many points projected above this level by distances varying from 2 to 14 inches.