Griswold.] 416 [Dec. 19, 
fine particles of novaculite can be ascribed to a mechanical origin.^ 
Thus the Arkansas novaculite would equal a slate minus the 
argillaceous component ; and impure novaculites are found with 
slates, as in the Boston Basin Avhere they have been collected by 
Mr. J. B. Woodworth. In Arkansas the argillaceous rocks of the 
series are shales, and the series offers all gradations between 
clayey strata, used in the manufacture of bricks, through siliceous 
shales to the novaculites. The theory of mechanical origin is 
therefore preferred to that of direct chemical precipitation. 
The pure silica of the novaculites might have been derived 
from sea water by the agency of certain organic forms, as has 
been demonstrated for some siliceous strata elsewhere,^ but a 
careful search with the microscope has failed to show signs of 
such organisms in the rock, and it is not probable that the traces 
of life have been obliterated by metamorphism or dynamic action. 
The whole series of strata which includes the novaculites has 
undergone the same strains, yet in the fossil horizons the organic 
forms are not noticeably distorted ; so while there has been an 
abundance of small movements along joints, there is no reason for 
believing that such action extended to particle on particle. 
Metamorphism of some sort has been the most frequent sug- 
gestion for at least a partial explanation of the origin of the 
novaculites ; and if reasons can be given for believing in the 
absence of much metamorphism it will not be necessary to discuss 
the various hypotheses in detail. The geological occurrence of 
1 In a paper entitled " Observations upon tlie erosion in the hydrographic basin of 
the Arkansas River above Little Rock" ( Ann. rept. Ark. geol. surv. for 1891, vol. 2, 
p. 153-166 ) , Dr. J. C. Branner gives the results of the observations of a year upon the 
material transported ia solution and suspension by the Arkansas River. The coarser 
solid matter pushed along on the stream bed was not included, so the results have a 
stronger bearing on the point at issue. It is here stated (p. 162) : — "Taking the 
observations of the entire year under consideration, the matter in solution is equal 
to about .31 of that in suspension. . . ." Analyses of filtered river water taken at 
different stages of volume in the river contained 1.83 and 11.81 per cent of silica. 
The sediment carried in suspension shows " sand and insoluble matter " 85.18 per 
cent, " soluble matter " 14.82 per cent. A complete analysis of the sediment col- 
lected with six liters of water is silica 69.53, alumina 11.65, iron oxide 4.46, carbonate 
of lime :6.62, carbonate of magnesium 3.52, potash .66, soda 1.14, organic and volatile 
matter 2.95, total 100.58. Thus the quantity of silica carried by the Arkansas River 
in solution is minute as compared with that in mechanical suspension. To generalize 
upon these facts and say that all skites and shales are made up of mechanical and 
chemical silica in proportions similar to those of the above river materials would be 
quite unreasonable, yet the writer feels a strong support in these observations for a 
belief in preponderating mechanical silica in such rocks. 
2 Ann. rept. geol. surv. Ark. for 1890, v. 3, p. 178-180. 
