vapor and fumes arose from several volcanic vents, and at frequent intervals there were showers of fine ash, while more rarely molten lava escaped and flowed down the slopes into the lake. During the larger eruptions, as has been recently witnessed at Martinique, there were violent gusts of wind, and blazing cinders of various sizes fell on all sides. Green branches were torn from the trees, and are found with the leaves still attached in the shales; while many leaves are burned and torn, and an abundance of charcoal testifies to the existence of forest fires. The clouds of fine ash bore to the ground all winged insects and, when falling in the shallow water of the lake, gave rise to layers of soft mud and sand, which, under the pressure of subsequent deposits, solidified into rock. In this way the shales were formed, the best being under heavy flows of mud or lava, which compressed them and permitted the preservation of the remains they contained, before decomposition had gone too far. Within a limited area hot waters strongly charged with silica surrounded and bathed the remains of the redwood trees, with the result that these are now wonderfully preserved as fossil stumps, one of them of great size.
In the course of ages, after the lake had disappeared, the streams flowing through the valley cut out the soft shales and carried them in fine particles to the Platte River, whence they found their way toward the plains. It is sad to think of the thousands of magnificent fossils which must have been thus destroyed—the infinitesimal fragments of which are now scattered far and wide over the Colorado plains, or have traveled perhaps to the Mississippi and the sea. The result of all this destruction, however, has been favorable to the paleontologist in this sense, that it makes the shales readily accessible at many points. Along the sides of the valley, close to the old shore lines, the fossil-bearing layers are plainly visible, and no doubt those which are preserved are far richer than were those which occupied the middle of the lake. The amount of the deposit still remaining is not known, but it must be very great, so that its possibilities could not be more easily exhausted than those of Herculaneum. Only a few places have been worked for any length of time, while small outcrops, inviting investigation, are very numerous. The work of uncovering the fossils is necessarily very slow. First of all, the heavy cap of solid rock has to be removed—and the farther one goes into the hillside, the greater it is—and then the shale has to be split with a knife into fine layers, often with great difficulty. With the utmost care, it is certain that many things will be lost, either from not being observed when uncovered, or from the shale not splitting in the right places. It would not be practicable to have the work done by untrained laborers, with the exception of the preliminary digging and shoveling, for they would destroy and lose far more than they found. Frequently the most precious and perfect insect remains are
