386 MK. F. RUTLEY ON THE ORIGIN OF [Aug. 1 894, 



of novaculite usually known as Arkansas stone' have been produced, 

 not from sandstones or quartz, but by the metamorphism of chert." l 

 Dr. Brauner seems to have neared what I believe to be the truth. 

 Many other opinions are quoted by Mr. Griswold, some of them 

 conflicting. His own views are given in the following words : — 



" Still it might be possible that the Arkansas novaculites were 

 metamorphosed cherts, were it not that their microscopic examina- 

 tion disproved the idea of great metamorphism. The fine grains of 

 silica composing the groundmass of the novaculites are not cemented, 

 but seem to be merely jammed together, the tenacity of the stone 

 being due to the interlocking of the irregular edges of the grains. 

 In the Ouachita stone are perfect rhombohedral cavities, about 

 which the fine grains of silica are closely packed in a layer show- 

 ing orderly arrangement ; outside this layer the granules have no 

 orderly arrangement. In an early stage of the history of the rock 

 the rhombohedral cavities undoubtedly contained crystals of calcite 

 or dolomite ; in fact, these crystals must have formed while the 

 grains of silica were yet free to move about, that is, while the rock- 

 material was in a soft condition similar to that of mud or ooze on 

 the sea-bottom. That the silica-particles were free to move is 

 shown by the fact that those granules immediately surrounding the 

 crystallizing calcite arranged themselves with their long axes per- 

 pendicular to the faces of the calcite-crystals. Thus the calcite 

 crystallized out in the original slime on the sea-bottom, or in the 

 still plastic beds below the slime ; and it is noticeable that the 

 calcium carbonate formed the tiny separate rhombohedra uniformly 

 distributed through the rock, just as one would expect the fragmental 

 carbonate to be distributed in an original sedimentary deposit, 

 instead of collected together into concretions or into separate layers. 

 The fact that the carbonate of lime did not gather into layers 

 or concretions may indicate that the sedimentation was a rapid 

 one." 2 



From this view I dissent. I do not believe that such rocks are 

 sediments, but that they are siliceous replacements of dolomite or of 

 dolomitic limestone-beds. 



Furthermore, I can find no evidence that the siliceous granules 

 are always arranged with their long axes perpendicular to the walls 

 of the rhombohedral cavities. Sometimes they are, at others they 

 are not ; but, as a rule, there is, as Mr. Griswold points out, an 

 orderly arrangement of these granules around the margins of the 

 cavities, a circumstance which tends to confirm the belief that the 

 rhombohedra of dolomite existed before any deposition of silica took 

 place around them. It is, indeed, often hard to recognize which 

 is the longest axis of one of these siliceous granules, but there is 

 no doubt that there frequently is a very decided direction of elon- 

 gation in the grains bordering the rhombohedral cavities. On the 

 other hand, in these directions of elongation it is clear, as shown by 

 using the quartz-wedge, that the optical orientation is not always 

 the same. 



1 ' Whetstones and the Novaculites of Arkansas,' p. 176. 2 Op. cit. p. 187. 



