DIAMONDS IN ARKANSAS MISER AND ROSS 265 



of brown mica, angite, and a small quantity of pervoskite, magnetite, 

 and chromite. 



The volcanic breccia is a consolidated aggregate composed of vol- 

 canic fragments ranging from dustlike material up to pebbles half 

 an inch or more in diameter. The original minerals have been 

 largely altered to serpentine, but the breccia is essentially similar 

 in mineral composition to the " hardebank." The breccia is, how- 

 ever, diamond bearing, whereas the " hardebank " and the volcanic 

 tuff appear to contain few if any diamonds. The diamonds con- 

 stitute a remarkably small proportion of even the most productive 

 rock. It is interesting to note that at the great Premier mine, in 

 South Africa, where the yield is very accurately known, diamonds 

 constitute less than 1 part in 12,000,000 of the peridotite. The 

 breccia, being fragmental, has been much more thoroughly weathered 

 than the massive " hardebank," and the products of the weathering 

 are at most places a soft earth and at others a fairly soft rock. 

 These weathered materials show many shades of green, blue, and 

 yellow, and are known by the miners as " green ground," " blue 

 ground," and " yellow ground." 



The volcanic tuff was probably formed in the same way as the 

 breccia, and the original minerals were probably similar. It has, 

 however, undergone a different type of alteration; and though the 

 breccia is now largely serpentine, the tuff contains much chlorite, 

 which gives a distinctly blue color. 



The volcanic eruptions that brought about the formation of the 

 diamond-bearing beds differed from the more familiar type of 

 volcanic eruption in that there was probably little extrusion of lavas, 

 but the explosive force must have been stupendous. A pipelike cra- 

 ter was blasted through older rocks, and fragments of these rocks 

 were mixed with the shattered peridotite, thus forming the volcanic 

 breccia. The massive " hardebank " probably welled up into the 

 throat of the crater and cooled there without producing notable 

 flows. 



RELATION OF THE PERIDOTITE TO THE OTHER IGNEOUS ROCKS 



OF ARKANSAS 



The peridotite in Pike County is of the same age or nearly the 

 same age as the other igneous rocks of Arkansas, which consist of 

 nephelite, syenite, pulaskite, and related types of intrusive rocks and 

 which occur in four small separate areas in the eastern part of the 

 Ouachita Mountain region and in the northwest border of the Gulf 

 Coastal Plain. One of these areas is in the Fourche Mountain region 

 near Little Rock, Pulaski County, where the igneous rocks have 

 yielded bauxite deposits; a second is near Bauxite, Saline County, 



