Cretaceous era of the earth, 136, 143, 151, 152. Crystals, conditions of formation, 357. Dana, 131, 139, 140. Dark Regions, 122-125; thought to be seas, 110, 111; named in accordance, 110, 113; change in aspect cast doubt on marine character of, 113, 114; change in, considered seasonal, 115, 120, 127, 163, 164; marine character lost, 30, 115-118, 163, 164; vegetation tracts, 119-127, 163, 164, 168, 169, 170; below level of surrounding surface, 130, 164; former ocean basins, 120, 129, 131; latitudinal development in, 123, 124, 126, 127. Dawes, 21, 23, 249, 250, 268. Day, Martian, length of, 34, 160, 166. Desert regions of the earth, 13, 149-155; as observatory sites, 12, 13; help explain Mars, 16, 17, 156; color of, 149, 151; compared with color of Mars, 150, 163; vegetation in, 150; position of, 153, 154; due to winds, 154. Desertism, 16, 89, 153-158. Deserts (see Reddish-ochre regions). _Deuteronilus_, 259-261. Development of canals, latitudinal law of, 299, 302, 375; follows melting of polar caps, 302, 338-340; across equator, 373, 375. Devonian era of the earth, 141. Diaphragm, the great, 265. Diplopia, 196. _Djihoun_, narrowest double, 228-230; embouchure of, 219-220; connection with Luci Ismenii, 260, 262. Double Canals, first seen by Schiaparelli, 28, 192; impression of, 193, 204; two classes of, 224; require steady definition, 194; phenomena of, 194, 205, 208, 212, 213; physical bond between the constituents of, 226; connection with bays, 232; optical theories of, 196-203; not illusions, 195-203, 209; widths of, 205, 206, 221-224, 229, 230, 233; length of, 205; seasonal change in, 205; constituents of, 204; original line of, 216, 217; number of, 205, 209; list of, 210, 211; gemination period of, 212, 213; direction of, 234-236; zonal distribution of, 236-239, 370; distribution in longitude, 236; tropical phenomena, 239, 240, 241, 242; compared with single canals, 240; convergent, 240; avoid blue-green areas, 241; connect with blue-green areas, 242. Dust storms, 90, 165. Earth, tilt of axis of, 34; seasons on, 35; polar caps of, 38, 41, 44, 45, 51, 54, 69; rainfall on, 79; viewed from space, 340; vegetal quickening opposite to that on Mars, 344. Eccentricity of orbit, effect on seasons, 46, 48, 52. Elevations on limb, 96, 97; measurement of, 98. _Elysium_, white in, 75, 76. Eocene era of the earth, 144. Eopaleozoic era, 140. _Euphrates_, 221, 231, 249, 258-261, 266, 267, 316; continuously double, 213; curious relation to the Portus Sigaeus and Phison, 218, 219. Evolution, 362, 366, 367; planetary, 363, 364; advance in, dependent on environment, 145, 146. Exploration, polar, 54. Eye, relation to camera, 272-274.