Volume 16, pages 221-225, 1931

     Recently, the author was loaned for study purposes several specimens of the so-called "Zebra" rock, from near Argyle Station, East Kimberly, Western Australia, (Lat. 16° 15', Long. 128° 40'), by Mr. George L. English, consulting mineralogist of Ward's Natural Science Establishment of Rochester, New York. A similar rock has been noted by T. Blatchford near Braeside Station on the Oakover River near the previously referred to locality, but it was not found in place.¹

     The "Zebra" rock from Argyle Station has been mentioned, its general microscopic characteristics and occurrence described by T. Blatchford.² The author is very fortunate in having had the use of an unpublished abstract of a paper dealing with this interesting formation which was read by Mr. R. A. Hobson before the Royal Society of Western Australia on July 8, 1930. This abstract gives the probable theories which may account for the peculiar banding in the rock and a diagrammatic cross-section showing the stratigraphic portion and general field relations of the formation .³

     According to the diagrammatic cross-section by Mr. Hobson, the "Zebra" rock lies unconformably on the Pre-Cambrian of the region and directly below a limestone containing the small gastropod, Salterella hardmanni. Forms similar to this have been described by Hall and Wallcott and referred to the Lower Cambrian in North America. It would seem, therefore, from the conformable relation which exists between the bedded "Zebra" rock and the Lower Cambrian limestone above it, the unconformity between the base of the "Zebra" rock and the Pre-Cambrian, that the formation under discussion is probably of Lower Cambrian Age.⁴

    
FIG. 1. Typical "Zebra" rock showing cavities on weathered surface.

    
FIG. 2. Typical "Zebra" rock showing halos and dark bands.

     Thin sections of the rock cut through adjoining light and dark areas, and a light area between two darker ones, show several very interesting facts. The extremely fine grained nature of the rock is confirmed, the constituent particles being so small that identification can not be made with any degree of certainty. Quartz and chlorite occur in grains large enough to be readily recognized. There are two types of quartz grains, one which is identified by its characteristic "blick" on rotation under crossed nicols, and the second which exhibits wavy extinction. The wavy extinction may be due to the successive extinction of a compound grain as suggested by Grawe.⁵ It was first thought that these grains were orthoclase, but an examination of spectrograms of both the light and dark colored portions of the rock taken with a quartz spectrograph showed that there was no potassium present. Chlorite flakes are found oriented in parallel position and this arrangement is constant across both the light and dark bands. The only essential difference in mineral composition between the two portions of the rock is the presence of hematite in the darker areas. The hematite is very finely disseminated through the darker parts and appears to grade gradually into the lighter portions. What appears to be limonite in reflected light is scattered through the lighter parts of the rock in a manner similar to that of hematite in the darker portions. The cavities mentioned in the macroscopic description are empty and, show a slight concentration of hematite around them.

     ORIGIN OF THE ROCK AND BANDING

     The origin of the "Zebra" rock, aside from its peculiar banding which is secondary, has not been suggested. The microscopic examination of thin sections leads the author to believe that the rock was originally a very fine grained detrital sediment. The reason for this statement is the fact that in some of the fine grained Paleozoic sediments this association of detrital and secondary quartz, in a fine grained ground mass containing chlorite, is very common. As an example, some of the finer Upper Devonian shales of central New York may be cited. Blatchford has found volcanic tuffs and breccias interlaminated with the rocks of the region and it is very likely also that the "Zebra" rock may be a fine grained representative of such a volcanic series.

     There appear to be four explanations which may account for the very peculiar and regular banding noticed in the rock. They are: (1) original sedimentary banding of materials varying in composition sufficiently to give red and white colored bands, followed by deformation before complete solidification; (2) original markings due to crystallization of a fine grained flow rock, such as the leopardite from North Carolina (see Fig. 3 for comparison); (3) the result of infiltration of iron-bearing solutions into a uniformly white rock; or (4) due to the leaching of the hematite from a uniformly colored red rock.