1897.] ASHLEY — GEOLOGY OF ARKANSAS. 229 



I. Ouachita stones : those novaculites which '^ originally con- 

 tained a large percentage of lime." 



II. True novaculites : these are the characteristic rocks of the 

 mountains of the Ouachita system. They are believed by Mr. 

 Griswold to be mechanical sediments. They are nearly pure silica, 

 though "originally containing a small percentage of lime." In 

 color they are white, black, red, yellow, gray and mottled. 



III. Silicious shales, containing ''little lime, but with more 

 abundant clay." 



IV. Sandstones and quartzites. 



V. Chert. 



The finding of a large number of species of graptolites in the 

 silicious shales has shown that the novaculites are of Lower Silurian 

 age and probably belong to the lower part of the Trenton. 



Peti'ology of the Carboniferous Area. 



Sandstones. — Probably nine-tenths of the Carboniferous beds are 

 sandstones. ]Mr. Griswold has described a microscopic slide of 

 sandstone from Grindstone Ridge in 6 S., 20 W., which gives the 

 characteristics of a typical sandstone of the region.^ 



" This stone is coarse grained, light gray in color." It contains 

 only a small amount of '' iron and earthy material." 



''There are rounded as well as angular quartz grains, and a few 

 grains of decomposed feldspar are present. The quartz grains ap- 

 pear to have been eroded against and even into one another ; many 

 of them exhibit lines or striations, which probably result from pres- 

 sure. Since these lines are not continuous from one pebble to an- 

 other, and have apparently no relation to each other, they must 

 have been produced in some manner on the original grain and in 

 the original rock. Some of the quartz grains show secondary silica 

 added to the original grain to form crystal faces as described by 

 Irving in his article on quartzites.' One quartz fragment is filled 

 with very fine needles which may be rutile." 



The specimen is a trifle finer grained than the average sandstone. 

 The sandstones naturally grade into each other, but certain arti- 

 ficial divisions may be made for convenience. These rocks have 

 not been, as a rule, identified with any definite position in the 

 strata. 



'^Geol. Sitrv. of Ark., An. Rep. for 1890, Vol. iii, p. 141. 



"^ Fifth Annual Report of the United States Geological Sitrvey, pp. 221-223. 



