Page:Popular Science Monthly Volume 79.djvu/433

Rh Company, and the Davis groups. On the Cunningham claims several long tunnels have been run. At the McDonald mine on the Controller Bay group is a working drift more than 600 feet long. On the claims of the English Company there are three tunnels with a total length of more than 900 feet. On the Davis group is a tunnel whose length is more than 500 feet. During the summer of 1910 the Davis group was the only one in the field upon which systematic development work was being carried on.

The coals of the Bering River field are of good quality, and the tonnage is unquestionably great. But the probable amount of this coal which is available at present or will be available in the near future can not be stated with any degree of reliability. The conditions of occurrence are such that until extensive development has revealed many data at present unknown, an estimate of the available coal would be little more than a guess. In coal fields of somewhat simple structure fairly reliable estimates may be made of the available coal from a study of the outcrops, but in fields such as the Bering field, where the structure is complex, such estimates are of little value, and in fact may be harmful. The figures are likely to be overemphasized, and even misused, by those who are endeavoring to interest the investing public. In much of the Bering field the rocks are folded, faulted, jointed and crushed; the coal beds are known to vary much in thickness within short distances along both the strike and the clip; the coal beds in several places may be seen to change somewhat abruptly into carbonaceous shale, and in places they are intruded by igneous rocks. As yet, the beds in the different parts of the field have not been correlated, nor is