n6 POPULAR SCIENCE MONTHLY 



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, solidi- 

 fied 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 split- 

 ting in the right places. It would not be practicable to have the work 

 done by untrained laborers, with the exception of the preliminary dig- 

 ging and shoveling, for they would destroy and lose far more than they 

 found. Frequently the most precious and perfect insect remains are 



