54 Branner and Bracl'ett — Peridotite of Arkansas. 



of the dyke as it now stands. Even the soft inclusions from 

 the Cretaceous are unaffected. The great number of these 

 inclusions suggest that the injected mass was cooled by them 

 to such an extent that it was rendered incapable of producing 

 contact metamorphism even on a very small scale. 



The horizontally bedded Cretaceous strata do not appear to 

 be disturbed in any way whatever by the presence of this dyke 

 or even by that of the main body of peridotite. This little dyke 

 affords the principal evidence in regard to the age of these igne- 

 ous rocks. 



The Paleozoic exposure at this locality is the most southerly 

 one known in the state. The rocks are all sandstones or quartz- 

 ites, frequently false-bedded, and contain many so-called "f'ucoid 

 impressions." They are much fractured and jointed and occur, 

 for the most part, as irregular and angular blocks, and only at 

 the extreme southwest part of the exposure is it possible to de- 

 termine their dip satisfactorily. The dip moreover is not uni- 

 form either in amount or direction, the one measured being 

 26 ° southwest, and somewhat below the average. The exact 

 contact between the Paleozoic and the igneous rock is not 

 visible. 



The rocks of this group vary considerably from flinty green- 

 ish quartzites to light-colored and porous sandstones, but this 

 variation is no greater than one might expect to find in the var- 

 iable sandstones of the Lower Carboniferous to which these are 

 supposed to belong. Some of the quartzites are extremely hard, 

 but the appearance of freshly broken specimens shows that this 

 hardness is to be attributed to the indurating effects of weather- 

 ing, rather than to contact metamorphism. In some instances 

 the sandstones are of a light brown color and contain traces of 

 vegetable matter, though no recognizable forms have thus far 

 been discovered. In other cases they are tinged with green 

 coloring matter, probably due to the presence of chlorite. 



Inasmuch as it has been suggested that the South African 

 diamonds may have been generated by the metamorphism of 

 the carbon in the carbonaceous shales penetrated by peridotite, 

 I should add that no such phenomenon is suggested by obser- 

 vations at this locality or upon these rocks. 



The Post-tertiary wash so widespread in southwestern Arkan- 

 sas is thinly scattered about the foot of the ridge of peridotite. 

 Its cobbles and pebbles are of sandstone, quartz, novaculite, 

 and jasper, cemented here and there into a ferruginous con- 

 glomerate. The fragments are usually much water-worn, but 

 some of them are subangular, while in size they range from 

 that of one's head downward. Careful search was made among 

 this material for fragments of peridotite or serpentine, but none 

 was found. From the readiness with which this rock decom- 



