448 AMERICAN SPIDERS AND THEIR SPINNINGWORK. 



These insect bearing strata, as described by Dr. M. E. Wads worth, 1 are 

 brownish and grayish brown shales, being simply the finer material of the 

 tufas laid down in laminae of varying thickness and coarseness. This vol- 

 canic material has evidently been worked over by water; so far, however, 

 as can be judged by microscopic examination, when the water commenced 

 its work the material was in loose, unconsolidated deposits. That it was 

 thrown out as an ash, or rather deposited as a moya or mud flow near its 

 present location, is the most probable supposition. The deposition ap- 

 pears to have been gentle but comparatively rapid, for there is no sign of 

 violence or even of such decomposition as one should expect in slow dep- 

 osition ; and showers of ashes falling on still water or a lake, acting on 

 an unconsolidated tufa bank, answer best the conditions called for here. 



II. 



According to Lesquereux the numerous leaves of some of the species 

 of plants are not crumpled, folded, or rolled as if driven by currents, but 



flat as if they had been imbedded in the muddy surface of the 

 Manner bottom when falling from trees or shrubs along the border of 



the lake. As leaves, seeds, and other parts of a plant are al- 

 ment ways intermingled with the fossil insects and araneads, we may 



conclude that their entombment resulted from dropping along 

 with the leaves into the water. It will be remembered that many spiders 

 make their snares permanently among leaves, or within the inner sur- 

 faces of leaves, so that, when they are stripped from their stems by 

 violence or natural decay, they must often drift from the banks into 

 streams, and if overhanging the water drop directly therein. Others, like 

 our Insular and Shamrock spiders, dwell within nests of curled leaves, 

 and these would meet the same fate under like circumstances. It is not 

 now uncommon to see such nests overhanging the borders of streams or 

 woven among the foliage of plants in the immediate vicinity. Supposing, 

 as we have a right to do, the same habits prevailing in the Oligocene 

 period as the present, all these leaf dwelling species would have been ex- 

 posed to submergence in the ancient Florissant Lake, and, being imbedded 

 in the mud, some of them, at least, might be preserved. 



Lesquereux further believes that the deposition of the vegetable ma- 

 terials took place in the springtime and that the lake gradually dried 

 during summer. He bases this inference on the complete absence of hard 

 fruits, together with the presence of flowers, of unripe carpels of elm and 

 maple, and of well preserved branches of taxodium, which, in the living 

 species, are mostly detached and thrown upon the ground in wintertime 

 or early spring. If this were so, there would have been far fewer mature 

 spiders at that season, and the very young would be less likely to fossilize. 



1 Scudder, Paleon. Floriss. 



