JBranner and Brackett — Peridotite of Arkansas. 55 



poses, however, it could hardly be expected that such fragments 

 would be preserved for any great length of time. 



The sum of our evidence favors the hypothesis that this per- 

 idotite is a simple injection which took place about the close of 

 the Cretaceous through and between the Paleozoic strata, and 

 penetrating the lower Cretaceous beds, and that whatever its 

 relations to orographic movements may have been, it caused no 

 great direct disturbance either chemical or physical in the 

 beds with which it appears in contact. 



It naturally occurs to one that the Tertiary subsidence and 

 the intrusion of these igneous rocks are associated in some way ; 

 but which is the cause and which the effect, the facts to be 

 gathered at this locality do not indicate. 



The course of geologic events at this place as indicated by 

 the geology of the region was as follows : 



Time. Event. 



1. ( At the close of the Carboniferous, the rocks of that age 



Close of the Car- 1 wt *re flexed, lifted, and subjected to very extensive subaerial 

 boniferous. ( erosion. 



„ ( The southeast margin of the eroded land sank beneath the 



< ocean and the lower and Upper Cretaceous beds were depos- 

 Early Cretaceous. ( i te & aga inst and upon them. 



) The land was elevated and the Cretaceous beds exposed to 

 Close of the Cre- "i a brief period of erosion, 

 taceous. ( 



( The igneous rocks were ejected through the Paleozoic shales 

 4. j and sandstones and the clays and soft sandstones of the lower 



Earlv Tertiary. ! ( an( ^ upper ?) Cretaceous, and the land sank beneath the seas 

 ^ in which the Tertiary beds were laid down. 



5_ ( The Tertiary series was elevated and in the slow process of 



p 1 1 t - i passing through the beach condition, its soft beds were sub- 



™' ( jected to extensive erosion and denudation. 



6. j 



Quaternary. I Quaternary events, which need not be specified here. 



Of about a dozen known occurrences of crystalline rocks in 

 the state of Arkansas, the peridotite of Pike county offers the 

 best evidence of the date of its intrusion. All the other known 

 exposures are north of the Cretaceous area and in a region in 

 which metamorphism has been so general that every trace of 

 the paleontologic evidence of the age of the rocks penetrated 

 that may have existed has been entirely obliterated, and we are 

 therefore unable to determine by any evidence thus far col- 

 lected, the precise age of those beds, and are consequently un- 

 able to determine the age of the eruptives. 



The syenites of Little Rock are not Archaean as they have so 

 long been supposed, but are intrusions into Paleozoic rocks, 

 probably of Lower Carboniferous age. They are overlain, how- 



