Even while the first hole at a site is in progress one rarely need be in doubt as to whether the core is from a boulder or from bed rock. Should the core be exactly the same kind of rock as adjacent exposed ledge, and (which is the best possible corroborative indication) should the stratification in both cases show the same angle with the perpendicular, the chances are practically infinite that the core is from bed rock. Even if cores at very moderate depths show perfectly satisfactory rock, it is well to put one or more holes to a great depth as insurance against any possible deep-seated condition that should be known.
The vibration of the core barrel breaks the core at the slightest seam, and each piece recovered is in itself perfectly sound. The lengths of core show whether the rock is sound and massive, or seamy, although they are practically of no value as showing whether the seams are tight and harmless or open and objectionable. It will be observed (and in fact is to be expected) that the core will often wedge inside the barrel and itself grind upon the rock; fragments also will break from the core or drop in from the side of the hole and be reduced by grinding to such fineness that they will be washed away and occasionally account for large differences between feet penetrated and feet of core recovered.
It is of course better, though expensive, to draw the core often and avoid as much of this loss as possible.
The character, hardness, stratification, number of seams and depths between withdrawals should all be carefully considered in forming an opinion as to any discrepancy between penetration and length of core. Neither can any one interval or length be considered entirely apart from the adjacent ones or from the total, for the reason that at any withdrawal an appreciable or even considerable length of core may be left behind. Say the core may part or be broken 6 in. above the cutting edge, or drop that much before being gripped, the withdrawal under question will show 6 in. less than the truth and the next one may show the same amount in excess of the penetration. Obviously the total length of core recovered between the surface and any given depth cannot exceed that depth, although between any given depth and the bottom the core might show an excess.
Fig. 10. Section of tray to hold rock cores.