Grazing. Pinchot, _Primer I_, pp. 69-73, II, p. 73. Pinchot, _Agric. Yr. Bk._, 1898, p. 187 Coville, _For. Bull._ No. 15, pp. 28-31. Roth, _First Bk._, p. 130, 178. Insects. Comstock, passim. Hopkins, _Agric. Yr. Bk._, 1902, pp. 265-282. Roth, _First Book_, pp. 115-130. Howard, _Entom. Bull._, No. 11, n. s. Hopkins, Spaulding, _Entom. Bull._, No. 28. Hopkins, _Entom. Bull._, No. 48. Hopkins, _Agric. Yr. Bk._, 1903, pp. 313-329. Hopkins, _Agric. Yr. Bk._, 1904, pp. 382-389, Figs. 43-56. Pinchot, _Primer_, I, p. 73. Felt, N. Y. _State Museum Bull._, 103, Ent. 25. Hopkins, _Entom. Bull._ No. 32. Hopkins, _Entom. Bull._ No. 56. Hopkins, _Entom. Bull._ No. 58. Spaulding and Chittenden, _For. Bull._ No. 22, pp. 55-61. [Footnote A: For general bibliography, see p. 4.] CHAPTER VII. THE EXHAUSTION OF THE FOREST. The exhaustion of the forest in the United States is due to two main causes: (1) Fire, and (2) Destructive Lumbering. FIRE. It is not commonly realized that forest fires are almost entirely the result of human agency. When cruisers first began to locate claims in this country, practically no regions had been devastated by fire. Now such regions are to be seen everywhere. Altho lightning occasionally sets fire to forests, especially in the Rocky Mountains, the losses from this cause are trifling compared with the total loss. [Illustration: Fig. 104. Slash, Left in the Woods, and Ready to Catch Fire. _U. S. Forest Service._] _Opportunities for fire._ There are a number of facts that make the forest peculiarly liable to fire. Especially in the fall there are great quantities of inflammable material, such as dry leaves, twigs, and duff lying loose ready for ignition. The bark of some trees, as "paper birch," and the leaves of others, as conifers, are very inflammable. It follows that fires are more common in coniferous than in deciduous forests. After lumbering or windfalls, the accumulated "slash" burns easily and furiously, Fig. 104. Moreover a region once burned over, is particularly liable to burn again, on account of the accumulation of dry trunks and branches. See Fig. 107. Long dry seasons and high wind furnish particularly favorable conditions for fire. On the other hand, the wind by changing in direction may extinguish the fire by turning it back upon its track. Indeed the destructive power of fires depends largely upon the wind. [Illustration: Fig. 105. Forest Fire. _U. S. Forest Service._]