24 TERTIARY INSECTS OF NORTH AMERICA. 



ure, which when of a grayish or reddish browu color resembles woody fiber. Some of 

 the fragments contain ellipitical cells, and a few shards of water-clear glass free from 

 inclusions were seen. 



Many crystals, entire or broken, are scattered throughout the groundmass of the 

 tufa. These crystals belong to plagioclase, sanidin, oliviue, magnetite, augite, and 

 quartz. But little quartz was observed ; one crystal contained triehites and vapor 

 cavities. The triehites are the same as those commonly seen in the quartz of granite, 

 but this appears to have been derived from the lava. The feldspar contains inclu- 

 sions of base, glass, and microlites, and through these the rock from which the feld- 

 spar was derived can often be told. The augites have the characters of andesitir 

 augite. A little palagouite and one crystal of microline were seen. 



The groundmass of the tufa is composed of comminuted and decomposed material 

 derived from the lavas before described. In the groundraass trachytic and rbyolitic 

 matt-rial appears to predominate. 



This specimen was chosen for description, as it best represented the general char- 

 acters of the tufas. 



B. THE COAKSF.R DEPOSIT .JUST ABOVE THE SHALES. 



This is more coarsely fragmental than any of the others, and is composed of a yel- 

 lowish browu earthy groundraass, holding fragments of quartz, feldspar, basalt, etc. 

 Some of the fragments appear to belong to the older rocks, but none of them were 

 seen in the section. Under the microscope the tufa is similar to the first one described, 

 but its fragments are larger and sometimes better marked. Some kaoliuized feldspars 

 and a little biotite were seen. The hornblende in the audesite is in the usual broken 

 forms, with blackened edges. 



C. A SPECIMEN FROM FINER PORTION OF THE UPPER CONTORTED RKI>S. 



A yellowish earthy groundmass holding crystals and fragments of augite and feld- 

 spar. On one side is a layer of tine detritus, composed of the same material as the 

 grouudmass of the more coarsely fragmental portion. Its microscopic characters are 

 similar to those of A, except that its materials are more decomposed and sanidin is 

 more abundant. One kaoliuized feldspar was observed. 



D. THREE SPECIMENS OF THE INSECT-SHALES. 



These are browuish and grayish brown shales, being simply the finer material of 

 the tufas laid down in laminae of varying thickness and coarseness. One is very 

 thinly bedded. 



This volcanic material has evidently been worked over by water, but the conditions 

 can of course best be told in the field. So far, however, as we can judge by micro- 

 scopic examination, when the water commenced its work the material was in loose 

 nncousolidated deposits. That it was thrown out as an ash, or rather deposited as a 

 moija near its present location, is the most probable supposition. It seems then to have 

 been taken up by the waves and spread out as it is now found. The reason for this 

 opinion is that the fragments are not worn, as they would naturally be if they had 

 been derived directly from solid rock by water action, and the decomposition is not so 

 great as we should expect. The deposition appears to have been gentle but compara- 

 tively rapid, for there is no sign of violence or even of such decomposition as we should 

 expect in slow deposition ; and showers of ashes falling on still water or a lake acting 

 on an uucousolidated tufa bank answer best the conditions called for here. It is 



