LIMESTONES OE SOUTH DEVON. 387 



crystals (0*02 inch long) are left on dissolving Devonian Limestone 

 contaiuing the so-called Stromatopora concentrica from. Kingsteign- 

 ton, near Teignmouth. These are completely riddled internally, and 

 much excavated on their faces externally by irregular cavities." 



I do not, therefore, notice the occurrence of these crystals as new 

 to geolog}', but with the object of discussing their origin and the 

 nature of the inclusions which they contain. Prof. SoUas ascribes ^ 

 the crystals in the limestone of Caldon Low to the displacement of 

 the silica of sponge- spicules by carbonate of lime, and the subsequent 

 crystallization of the freed silica. Of the fact that micro-crystals of 

 quartz do, in some instances, originate from sponge-spicules I have 

 seen evidence in the Carboniferous Limestone of Clifton, but in the 

 Devonian Limestone that I have examined I have noticed no such 

 process, nor have I met with any siliceous organisms. The 

 micro-crystals of quartz that I have obtained from these rocks 

 appear to have originated from the silica of decomposing silicates, 

 but I am not prepared to say that there may not be exceptions. 

 I would suggest that the ' riddled ' appearance in the crystals from 

 Kingsteignton, to which Prof. Sollas refers, is really due to the 

 liquid inclusions that occur so numerously in the crystals to which 

 I have referred in this paper (PL IX. fig. 5). 



As to the nature of these inclusions I am in doubt. All the 

 evidence we have may be summed up by saying that the inclusions 

 occur in the siliceous and apparently micaceous flakes and masses 

 which ultimately give rise to the micro-crystals of quartz, and that 

 the liquids are enclosed by the latter. As I have before stated, I 

 regard the siliceous masses and flakes as the residuum of decomposed 

 siliceous minerals ; and if this be so, it is possible that the liquids 

 may be products of this decomposition. It is also of interest to 

 note that the crystals of quartz are most numerous in the Lum- 

 maton and Barton Limestones, i. e. in those which are most altered 

 by crystallization. 



EXPLANATION OF PLATE IX. 



Fig. 1. Insoluble Eesidue. Illustrates the formation of micro-crystals of 

 quartz. A siliceous mass is represented, contaiuing liquid inclusions 

 and becoming crystalline, the crystals retaining some of the liquid 

 inclusions. The siliceous mass is, apparently, the remnant of a de- 

 composed silicate. X 65 diam. 



Fig. 2. A portion of Fig. 1 X 230 diam. Shows the liquid inclusions. 



Fig. 3. Limestone from Hope's Nose, showing a tubular organic structure 

 suggestive of Alga?. X 65 diam. 



Fig. 4. Insoluble Residue. A siliceous paste crowded with microlitic needles. 

 From Hope's Nose Limestone, near Torquay. X 220 diam. 



Fig. 5. Micro-crystals of quartz containing liquid inclusions, originating from 

 the crystallization of siliceous material ; as illustrated by Figs. 1 & 2. 

 X 65 diam. 



Discussion. 



Dr. SoRBY said that he was probably the first to study the micro- 

 scopical structure of the Devonian Limestones of Devonshire, but 



^ Op. et loc, cit. 



