DIAMONDS IN ARKANSAS MISER AND ROSS 267 



altered grains of serpentine and fragments of peridotite. That the 

 peridotite is not younger than the Bingen is shown by the fact that 

 the Bingen rests upon the peridotite at the American mine and at 

 the Black Lick. 



COMPARISON OF ARKANSAS AND SOUTH AFRICAN ROCKS 



The peridotite of Arkansas is generally believed to be similar in 

 character and mode of occurrence to that of South Africa, but the 

 exact points of similarity have not been recorded. Wagner, 2 who 

 has carefully studied the South African diamond deposits, says of 

 them : 



The pipes represent deeply eroded, funnel-shaped volcanic necks of the 

 Maar type, which appear to have been formed by the violent explosive libera- 

 tion at the earth's surface of highly compressed vapors and gases, emanating 

 from a deep-seated, ultrabasic magma. They are occupied, as a rule, by 

 nonvolcanic detritus derived from the shattering and comminution of the 

 rocks pierced by the explosions, by fragmentary material derived from tritura- 

 tion of kimberlite [peridotite], and at greater depths by solid plugs of the 

 later rock. 3 



Dealing first with the relationship of pipes to fissures, we have learned that 

 the magma in its ascent appears invariably to have been guided to within a 

 greater or less distance of the original surface by planes of structural weak- 

 ness in the earth's crust. * * * The earliest eruptions appear to have been 

 in the nature of mighty explosions, which resulted in blowing out of funnel- 

 shaped apertures. The bulk of the material forcibly ejected during these out- 

 bursts no doubt fell back into the vents, which at one stage of their history 

 may thus have been more or less completely occupied by nonvolcanic detritus. 

 The relief of pressure occasioned by these earlier explosions must be assumed 

 to have led to the ascent of the magma into the pipes, where it appears, as a 

 rule, to have given rise, by successive eruptions, to a number of distinct 

 columns of kimberlite [peridotite] and kimberlite tuff. 4 



The material that occupies the vents Wagner describes as follows : 



It has been pointed out that in so far as the pipe filling is concerned, the 

 rocks pierced by the explosions, the kimberlite magma, and the atmosphere 

 have all contributed. 5 



We may divide * * * the foreign matter of the pipes into three principal 

 groups — rock fragments derived from the adjacent pipe walls; xenoliths [in- 

 cluded fragments] of rock which have been brought up from below ; masses 

 of rock which, to attain the position in which we now find them, must have 

 fallen into the pipes from above." 



The pipe rock proper consists of kimberlite and of material derived from 

 its brecciation, comminution, and decomposition. 7 



The microscope reveals the fact that among the products comprehended 

 under the general term of blue ground three main varieties may be distin- 



8 Wagner, P. A., " The diamond fields of southern Africa," 1914. 

 8 Op. cit. p. 5. 

 *Op. cit, p. 41. 



6 Op. cit. p. 18. 

 «Op. cit., p. 20. 



7 Op. cit., p. 24. 



