158 ZOOLOGY 



The mud or lava. At times .of eruption there were, no 



volcanoes doubt, violent gusts of wind and poisonous gases, while 

 hot cinders fell here and there and set fire to the forests. 

 Thus leaves and even branches were torn from the trees, 

 and charcoal may still be found to testify to the forest 

 fires. Insects and other creatures were killed, and fell 

 into the shallow water of the lake, where they were 

 presently covered by deposits of the finest ash, falling 

 gently from above. Thus the various remains were hid- 

 den beneath successive layers of volcanic material, and 

 when a mass of lava flowed over the whole, its weight 

 pressed the wet ash down, and in course of time con- 

 verted it into hard shale. What had been the life of 

 the locality, now crushed flat, was hermetically sealed 

 between the layers, to be uncovered in about a million 

 years by creatures of a kind not then in existence. 

 Little could the stray butterfly, perishing miserably, 

 realize that some day its remains would be placed in a 

 museum, where they would be the wonder and admira- 

 tion of many generations of men ! 



How the 3. In the course of ages the volcanoes ceased their 



activities, and movements of the earth drained the lake. 

 The climate became much cooler and drier, and the 

 fauna and flora changed accordingly. Whatever de- 

 scendants of the old Florissant plants and animals might 

 exist mostly migrated to quite other parts of the country, 

 though some doubtless still live in Colorado. For ex- 

 ample, the narrow-leafed cottonwood of the foothill 

 gulches is so similar to that common in the Florissant 

 shales, that we can hardly doubt that the former has 

 been derived from the latter. Streams running through 

 the valley bottom cut into the soft shale, and enormous 

 quantities of it were carried away to the rivers of the 

 plains and perhaps even to the sea. What precious 



