J. 8. Diller — Origin of Paleotrochis. 341 



nodule. The biconical form of Paleotrochis suggests that it 

 originated as two spherulite sectors of which the apices were 

 the centers from which the libers radiated. This would seem 

 to be the simplest way to account for the most regular as well 

 as many of the irregular forms, but of the specimens examined 

 I have not been able to find one that certainly originated in 

 that way. 



A number of the fossil forms with a well-marked cup in the 

 exposed apex turned out to be flat hemispherical or thin len- 

 ticular in section, and are composed wholly of spherulitic fibers. 

 Although admitting much irregularity especially on account 

 of the supposed gemmiferous character of Paleotrochis, the 

 ones which have been considered the most characteristic of the 

 fossil are the distinctly biconical forms. These so far as seen 

 are chiefly granular quartz with more or less green biotite. 



It is important to note also that the dark groups of green 

 biotite occur in the interior of very irregular nodules which 

 have ro suggestion in them of Paleotrochis. Irregular flat- 

 tened lenticular masses of granular quartz with green biotite 

 occur within the spherulites as well as about them. The 

 green mica is found only in the most coarsely granular groups 

 of quartz. 



The following chemical analysis, made by W. F. Hillebrand, 

 shows that the rock has the composition of a rhyolite and 

 accords closely with the results of the microscopical study. 



Analysis of the Paleotrochis-bearing rock of Sam Christian 

 Mine.* 



Si0 9 .- 79-57 



Ti0 2 -11 



A1 2 3 11-41 with a very little P 2 5 



Fe 2 3 -20 



FeO -70 



MnO none / 



CaO) 



SrO f 21 



BaO -05 



MgO a very little 



K 2 3-52 



Na 2 - 3-46 



H 2 below 105°.... -18 

 " above " "61 (ignition) 



100-02 



, Recognizing the Paleotrochis rock as an acid volcanic full of 

 spherulites, it is easy to understand the great variation in the 

 form of the nodules. Such rocks are in many places distinctly 

 banded and were long considered siliceous sediments, but by 



* No other constituents looked for. 



