No.2.] SCUDDER ON GEOLOGY OF FLORISSANT, COLORADO. 287 

 B.— The coarser deposit just above the shales. 



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

 lowish brown earthy groundmass, holding fragments of quartz, feldspar, basalt, «fec. 

 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 kaolinized feldspars 

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

 forms, with blackened edges. 



C. — A SPECIMEN EEOM FINER PORTION OF THE UPPER CONTORTED BEDS. 



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

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

 groundmass 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 kaolinized feldspar was observed. 



D. — Three sPEcrivrENS of the insect-shaxes. 



These are brownish 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 

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

 as a moya 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 isthat the fragments are not worn as they would naturally 

 be if they had been derived directly from solid rock by water action, and the decom- 

 position is not so great as we should expect. The deposition appears to have been 

 gentle but comparatively rapid, for there is no sign of violence or even of such decom- 

 position as we should exiDect in slow deposition ; and showers of ashes falling on still 

 water or a lake acting on an unconsolidated tufa bank answer best the conditions called 

 for here. It is probable from the kaolinized feldspars and the macroscopic fragments 

 of apparently older rocks that the latter are present in the tufa to some extent. This 

 can best be explained by the supposition that it was deposited as a moya or mud- 

 flow within reach of the waters that have worked it over and deposited it in its pres- 

 ent position. As we said before, the field evidence must be relied upon mainly in 

 deciding such questions as these. 



M. E. WADSWORTH. 



Cambridge, Mass., Jjiril 15, 1880. 



Another section, less carefully measured and noted with less detail 

 than the other, was taken at or near the same place as Dr. Peale's, men- 

 tioned at the beginning of this article, viz, at the extremity of one of 

 the promontories jutting in a southwesterly direction into the middle of 

 the upper chain of lakes, just west of the school-house* and about three 

 kilometers west of Gastello's Banch. The top of the hill was covered with 

 granitic gravel and loose bowlders of dark scoriaceous trachyte; below 



*Not the school-house before mentioned, which lies to the south of Castello's Ranch. 



