Geology of the Paleozoic area of Arkansas south of the novaculite region / by George H. Ashley.

  • George Hall Ashley
Date:
1897
Catalogue details

Licence: Public Domain Mark

Credit: Geology of the Paleozoic area of Arkansas south of the novaculite region / by George H. Ashley. Source: Wellcome Collection.

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    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. Petrology 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. Surv. of Ark., An. Rep. for 1890, Vol. iii, p. 141. '‘■Fifth .Annual Report of the United States Geological Survey, pp. 221-223.
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