fore, which favor the formation and escape of these gases, favor the formation of a coal which is high in uncombined or "fixed" carbon.

Conditions which favor the formation and escape of the gases formed by the decay of wood out of ordinary contact with the atmosphere are: Pressure, heat, folding and erosion of the containing strata, which allows the escape of the gases, and time, which is necessary for the chemical transformation, ordinarily slow.

In accordance with these conditions, coal in the older formations and in regions of tilted strata often contains a very high percentage of fixed carbon and is classed as anthracite. Where the formation is young but the region one of tilted strata, and of crushing, bituminous coals are common; whereas coal which, in the geological sense of the term, is recent, and forms part of a horizontal series that has not been subjected to great pressure. or folding, is nearly certain to have a comparatively low percentage of fixed carbon. These are practically, the conditions under which the lignites are found."

Fixed Carbon.-The average amount of fixed carbon in twenty-six samples of North Dakota lignite analyzed in 1900, when thoroughly dried, was 52 per cent. Sixty samples analyzed this year, after thoroughly drying, gave 51.21 per cent of fixed carbon. Compilations given in a former report of this Survey* show, for forty-one samples of West Virginia bituminous coal, 67.16 per cent of fixed carbon; for twenty-two samples of the better grades of semibituminous dry coals of Maryland, 75.99 per cent, and for twenty-six samples of dry bituminous coal from Pennsylvania, 67.97 per cent. Judged by the fixed carbon alone the heating power of one ton of North Dakota lignite is worth 76 per cent of a ton of West Virginia bituminous; 65.9 per cent of Maryland semibituminous, and 74.4 per cent of Pennsylvania bituminous. These figures show that the North Dakota lignites rank far above the coals that are the average representatives of this class.

Comparing the lignites of North Dakota with the product of the west interior coal fields, which includes Iowa, Missouri and Kansas, with respect to fixed carbon, the lignite appears to be

* First Biennial Report, E. J. Babcock, pp. 81, 82, 83.

its equal. Taking the recent United States report* and computing them on a dry basis, the average amount of fixed carbon for twelve samples is about 50 per cent. These coals, however, give a slightly higher evaporative test than do the lignites. By both standards, however, the lignite appears to be practically equal, when properly burned, for steam purposes, to the coals of Iowa, Missouri and Kansas.

Volatile Matter.-The average amount of volatile matter computed from sixty analyses of different lignite coals is 35.63 per cent. When the fixed carbon alone is considered as determining the fuel value of a coal, the worth of the volatile matter is overlooked. Under proper conditions these gases may be made to develop a great deal of heat, and no appliances which aim at an economical combustion of lignite can afford to disregard

them.

The Percentage of Ash in the sixty samples analyzed this year is generally low, the average amounting to 8.5 per cent. Compared with the coals of Iowa, Missouri and Kansas regions, the lignites are notably purer. An average of twelve samples described in the report on this area already quoted gives 10.1 per cent of ash. In the ash from some North Dakota coals there is a tendency to form clinkers, due to the presence of clay, but generally the lignite burns like wood, leaving a gray, powdery residue.

Sulphur. The percentage of sulphur in the lignites is usually low, much lower than in most bituminous coals. The following samples are fairly representative:

The sulphur in these analyses was determined in dry coal, which brings the percentage one-fourth higher than for lignite fresh from the mine.

A yellow powder, composed of calcium sulphate and iron oxide, occurs often in the upper part of lignite seams, and in color and texture resembles sulphur.

* The Western Interior Coal Fields, H. Foster Bain, Extract from the Twenty-second Annual Report, 1900-1901, part III.