Cannel, boghead, torbanite, oil shale

The gradational interrelationship of coal with cannel, boghead, torbanite, and oil shale is reemphasized in view of contrary opinions expressed recently. An example from South Africa that has recently been studied in some detail is cited in illustration of how closely coal and torbanite may be related. The scientific classification of the carbonaceous rocks requires a broader basis of comparison than is generally adopted; authoritative statements cannot have very much weight unless supported by more correlated data than exist now.