﻿244 Geological Society, 



in the upper surface of the stone. He further discusses the com- 

 position of the stone, and describes experiments which he made to 

 illustrate his views. 



3. 'On the Discovery of Molluscs in the Upper Keuper at 

 Shrewley, in Warwickshire.' By the Rev. P. B. Brodie, M.A., 

 F.G.S. 



March 21st. — Dr. Henry Woodward, P.R.S., President, 

 in the Chair. 



The following communications were read : — 

 1. ■ On the Origin of certain Novaculites and Quartzites.' By 

 Prank Rutley, Esq., P.G.S., Lecturer on Mineralogy in the Royal 

 College of Science, London. 



The novaculites of Arkansas have already been admirably 

 described by Mr. Griswold in vol. iii. of the Arkansas Survey 

 Report for 1890. One of the characteristic microscopic features 

 in Ouachita stone is there stated to consist in the presence of 

 numerous cavities, often of sharply-defined rhombohedral form, 

 which Mr. Griswold considers to have been originally occupied by 

 crystals of calcite or dolomite. 



The Author, while admitting that the cavities were no doubt 

 once filled by the latter mineral, ventures to differ from Mr. Griswold, 

 and some of the authorities he cites, concerning the origin of the 

 rock. Crystalline dolomites, when dissolving, become disintegrated 

 into minute but well-formed rhombohedra. As the process of 

 dissolution proceeds these crystals may become so eroded that 

 the rhombohedral form is no longer to be recognized. The Author 

 points out that no inconsiderable proportion of the cavities in 

 Ouachita stone present irregular boundaries, such as the moulds of 

 partially eroded rhombohedra would show. He then offers a 

 fresh interpretation of these cavities, so far as the origin of the 

 rock is concerned : — 



1st. He assumes that beds of crystalline magnesian limestone 

 have been slowly dissolved by ordinary atmospheric agency and the 

 percolation of water charged with carbonic acid or other solvent. 



2nd. That, as the limestone was being dissolved, it was at the 

 same time being replaced by silica, which enveloped minute isolated 

 crystals and groups of crystals, some perfect, others in various stages 

 of erosion. 



3rd. That the silica assumed the condition of chalcedony, its 

 specific gravity, as stated by Mr. Griswold and as determined by 

 the Author, being low in comparison with that of quartz. 



4th. The residuum of the original dolomite or dolomitic limestone 

 was removed, leaving the perfect and imperfect rhombohedral 

 cavities. 



A calciferous, gold-bearing quartzite from the Zuluiand gold- 

 fields is described and a similar origin is ascribed to it, but in 

 this case the original rock appears to have been simply a limestone, 



