Vol. 50.] CERTAIN NOVACTXLITES AND QTJARTZITES. 391 



[Postscript. — On perusal of the remarks made by Dr. Hinde, in 

 the discussion which followed the reading of this paper, it would 

 appear that, in stating the thickness of the novaculite-formation 

 with its shales and sandstones, he lost sight of the fact that the 

 novaculites themselves occur in beds which, according to Mr. Gris- 

 wold, range from only a few inches up to 15 feet in thickness. 

 There is nothing improbable in the erosion or replacement of lime- 

 stone-beds of such dimensions. With regard to the question : What 

 became of the limestone ? it can only be answered that, if once 

 there, as I believe it to have been, it was carried away in solution : 

 the usual fate of limestones. As for the silica, it is difficult to 

 express any decided opinion concerning the source from which it 

 was derived. It may have been deposited from thermal waters. — 

 May 23rd, 1894.] 



EXPLANATION OF PLATE XIX. 



Fig. I. Arkansas stone. X 380. Crossed nicols. 



Fig. 2. Flint-pebble, Thames Gravel. X 380. Crossed nicols. 



Fig. 3. Ouachita stone. The dark patches are spaces due to cavities, often of 



distinctly rhombohedral form, once occupied by crystals, probably of 



dolomite. X HO. Crossed nicols. 

 Fig. 4. Siliceous rock, approximating to quartzite and containing rhombohedral 



crystals of a carbonate, probably dolomite. From conglomerate. 



Purtiall, Deccan, India. X 140. Crossed nicols. 

 Fig. 5. Magnesian limestone, occurring in the Carboniferous Limestone Series. 



Cumberland Cavern, Matlock Bath, Derbyshire. X 140. Ordinary 



light. 

 Fig. 6. Rhombohedra of dolomite derived from the preceding specimen by the 



action of dilute hydrochloric acid. X 140. Ordinary light. 

 Fig. 7. Pebble from conglomerate, Purtiall, India. Edge of the same section 



as that represented in fig. 4. X 380. Crossed nicols. 

 Fig. 8. Auriferous quartzite, Kondweni, Zululand. Edge of quartzose portion 



of section. X 380. Crossed nicols. 

 Figs. 9 & 10. Forms of cavities in Ouachita stone, once occupied by partially 



eroded crystals of dolomite. X 75. 



Discussion. 



Dr. G. J. Hinde complimented the Author on the ingenuity of his 

 explanation of the origin of the Arkansas novaculite-rocks, but failed 

 to see that there was any evidence for his fundamental assumption 

 that these rocks were originally limestones or dolomites which were 

 now replaced by silica. Taking into account that the novaculites 

 were over 500 feet in thickness, and that they were interbedded with 

 shales and sandstones, it might reasonably be asked what had 

 become of the limestone of which they were originally supposed to 

 be formed, and also whence the silica had originated which was 

 stated to have replaced the limestone ? 



The speaker did not agree with Mr. Griswold that the novaculites 

 were produced by simple sedimentation of fine fragmental silica ; 

 but he considered that their structure of cryptocrystalline silica, 

 and their resemblance to Chalk-flints in microscopic characters, 

 which Mr. Kutley had pointed out, indicated that, like these latter, 



