198 ME. E. WETHEEED ON IIS'SOLIJBLE EESIDUES rRO]U. 



EXPLANATION OF PLATE YIII. 



Fig. 1. Detrital quartz, from the Lower Limestone Shales. x40 diam. 



2. Another residue from the Lower Limestone Shales, consisting of 

 detrital q\;artz, siliceous pseudomorphs of portions of Crinoids 

 and amorphous silica. X 40 diam. 

 ■ 3. Micro-crvstal (A) and a fragment of amorphous silica (B) passing into 

 the chalcedonic or crypt o-crystalline and the crystalline conditions 

 at the outer portion. From the Middle Limestone. X 80 diam. 

 In the centre of the crystal is a nucleus consisting of a grain of 

 detrital quartz surrounded by a substance having the structure 

 of amorphous silica. 



4. Eesidue from the Middle Limestone, consisting of micro-crystals of 



quartz with detrital quartz-nuclei. x80 diam. 



5. Eesidue from the Middle Limestone, consisting of grains of detrital 



quartz, chalcedonic spicules of sponges, and other fragments of 

 chalcedonic silica. x40 diam. 



6. Eesidue from the upper portion of the Middle Limestone, consisting 



of detrital quartz, amorphous silica in very small fragments, and 

 chalcedonic spherical bodies, the nature of which ^is not clearl 

 known. x40 diam, 



DiSCFSSION. 



The Peesident observed that while the fact of the occurrence of 

 quartz-crystals in limestones was well known, a number of points 

 had been brought forward by the Author for the first time. It was 

 clear that the several members of the Carboniferous-Limestone series 

 must have undergone changes of different kinds, but some of the 

 differences observed might be due to peculiarities in the original com- 

 position of the rock. 'J'he existence of nuclei of fragmental quartz in 

 the crystals, such as had already been described in certain sand- 

 stones, and the indications of a gradual passage of amorphous silica 

 into chalcedony and so into quartz had not before been noticed. 



Prof. Bonnet said that the President had already touched upon 

 the salient points of Mr. Wethered's investigations, so that there 

 was little more for him to do than to express his sense of their 

 value ; among other things the paper was interesting as throwing 

 additional light on the formation of flints and cherts. He believed 

 that the sponge-spicules indicated the source of much of the silica. 

 There might also have been chemical precipitation, though not 

 directly, from the water whicb had taken up silica from more 

 soluble organisms in percolating the rock ; so that, as it were, the 

 more powerful organism steals the silica from the weaker ones. 



Dr. HiNDE was obliged to the Author, whose work confirmed his 

 own views, and had been executed in a careful manner. Silica in 

 the rocks;is constantly passing towards the stable condition of quartz. 

 He should like to know what was the evidence of the amorphous 

 character of the silica in the centre of the quartz-crystal. He re- 

 ferred to the various stages of erosion which may be observed in 

 sponge-spicules, prior to final solution, which in many cases has 

 taken place. 



Mr. Etheeidge also complimented the Author, and spoke of the 

 importance of determining the species of the Foraminifera together 



