Vol. 50.] ORIGIN OF NOVACULITES AND QUARTZITES. 377 



21. On the Origin of certain Novaculites and Quartzites. By 

 Frank Rutley, Esq., F.G.S., Lecturer on Mineralogy in the 

 Royal College of Science, London. (Read March 21st, 1894.) 



[Abridged.] 



[Plate XIX.] 



Although various explanations have been offered regarding the 

 origin of certain novaculites it seems that more may yet be said 

 upon this subject. A useful resume of tbe opinions held by different 

 authors is given in the ' Annual Report of the Geological Survey of 

 Arkansas for 1890,' 1 by Mr. L. S. Griswold. Before attempting 

 to explain my own views it seems desirable that I should allude 

 briefly to the specimens upon which those views are based, and 

 especially to the Arkansas novaculites, since Mr. Griswold's intimate 

 acquaintance with these rocks, both in the field and in the labora- 

 tory, lends great weight to his opinions. Indeed, it may seem 

 presumptuous on my part to question them, since I have no know- 

 ledge of the ground with which he is so familiar, and also because 

 the material at my disposal consisted of two small specimens, while 

 he has had mountain-ranges to work upon. 



The specimen of Arkansas stone whicb I have examined is a 

 bluish-white rock with a faint yellowish tinge, translucent in 

 moderately thick splinters, breaking with a conchoidal to splintery 

 fracture and bearing a general resemblance to chalcedony. The 

 structure is cryptocrystalline, and it shows under the microsuope 

 the outlines of what were once cavities of rhombohedral form, now 

 occupied by siliceous deposits of secondary origin. 2 The specimen 

 of Ouachita 3 stone resembles a faintly yellowish-white or cream- 

 coloured biscuit-porcelain, except that it exhibits barely a trace of 

 translucency on thin edges. 



Under the microscope this rock is also seen to possess a crypto- 

 crystalline structure, but of somewhat coarser texture than that of 

 the Arkansas stone. It likewise shows numerous little cavities, 

 some of which have irregular boundaries, but a large proportion of 

 them have the form of rhombs and represent cavities once occu- 

 pied by crystals of a rhombohedral carbonate (PI. XIX. fig. 3). 

 The siliceous grains which form the walls of these cavities often 

 show a regular arrangement, like a course of masonry (PL XIX. 

 fig. 3) ; but beyond these borders the arrangement of the siliceous 



1 Vol. iii. ' Whetstones and tbe Novaculites of Arkansas.' I am greatly in- 

 debted to Mr. W. Topley, F.R.S., for calling my attention to this admirable 

 work, and also for tbe loan of tbe book. 



2 This specimen appears to correspond with tbat described as ' Slide No. 31 ' 

 of Mr. Griswold's series. Op. cit. p. 135. 



3 Usually spelt ' Wasbita ' in this country. I adopt Mr. Griswold's spelling 

 as doubtless the more correct. 



