72 PKOCEEDINGS 0E 1HE GEOLOGICAL SOCIETY. [May 1894, 



grains of felspar, tourmaline, and zircon. In the main mass of 

 the calcareous rock there is, of course, a less amount of detrital 

 quartz, hut the presence of micro-crystals of quartz, as also of 

 amorphous and chalcedonic silica and of sponge-spicules, was in- 

 dicated. The nature of the amorphous and chalcedonic silica in 

 the limestone, and the relations of this silica to the small quartz- 

 crystals, were also discussed. The latter were shown in some in- 

 stances to possess nuclei of detrital quartz, and, where this is not 

 the case, to have resulted from the crystallization of amorphous 

 silica. The chief interest in this portion of the paper, as was 

 remarked at the time, lay in the indications of a gradual passage 

 from amorphous silica into chalcedony, and so into quaitz, it being 

 further observed that silica, in the rocks, has a tendency to pass- 

 towards the stable condition of that mineral. 



Devonian. — In direct continuation of the last subject, I have to 

 refer to another suggestive paper by Mr. "Wethered on the Devonian 

 Limestones of South Devon. In drawing conclusions from the 

 examination of the insoluble residues of examples collected in the' 

 neighbourhood of Torquay and elsewhere, he observes that, whilst 

 well-rounded grains of detrital quartz were found in the Carboni- 

 ferous Limestono of Clifton, no such detrital grains can be discovered 

 in the Devonian Limestone residues examined by him. Further, in 

 discussing the occurrence of micro- crystals of quartz he refers to 

 an observation by Prof. Sollas that such crystals are left on dis- 

 solving Devonian Limestone, containing the so-called Stromatojpora 

 concentrica, from Kingsteignton. Yet Mr. Wethered doubts the 

 organic (sponge-spicule) origin of these micro-crystals of quartz in 

 the Devonian Limestones, since he has not met with any siliceous 

 organisms, nor noticed any such process as that described in his 

 previous paper. He is inclined to believe that these micro- crystals 

 of quartz have originated from the silica of decomposing silicates, 

 and, as a case in point, he notes that the crystals of quartz are the 

 most numerous in those limestones which have undergone the 

 greatest amount of alteration through crystallization. 



Mr. Wethered allows that the conclusions drawn from the micro- 

 scopic examination of the Devonian Limestones are not very satis- 

 factory, so far as structure is concerned. Yet he has obtained 

 ample evidence that these limestones have been built up by the 

 remains of calcareous organisms, though the outlines of structure 

 have, for the most part, been obliterated by molecular changes. It 



