ANNIYEESAEY ADDEESS OF THE PEESIDENT. 6^ 



fig. 2, which exhibits all the above-named facts somewhat unusually 

 well. The original nucleus and the deposited quartz are always 

 in crystalline continuity ; so that, when rotated in polarized light, 

 both turn dark and light simultaneously. 



The contrast between the original nuclei and the subsequently 

 deposited quartz is also very well seen in the case of a specimen of 

 the New Eed Sandstone from Penrith, kindly given to me by 

 Prof. Morris, since the nuclei are stained red whilst the deposited 

 quartz is colourless, as may be seen at once by examining the sand 

 in benzol. We thus also learn that the reddening of the grains 

 took place before the deposition of the quartz. 



As will be readily understood, if the original nucleus were com- 

 posed of perfectly limpid quartz, it would be impossible to distin- 

 guish between it and the subsequent deposit. Possibly this is why 

 I can detect no nuclei in the crystallized oolitic sand at Scar- 

 borough, and thus there is no positive evidence that the whole 

 was not deposited chemically. 



The source of the quartz thus chemically deposited on the 

 grains of sand is by no means clear. In some cases it may be 

 derived from the decomposition of associated felspar ; but much 

 decomposed felspar may be present without any such deposition. 

 Possibly some of the more striking examples are due to the special 

 local action of heated water, as shown by M. Daubree in the case 

 of the Vosges sand. 



I have met with crystallized sand in deposits of various ages, 

 from the Oolites down to the Old Eed. Most of the specimens 

 were very friable, and the grains but little coherent. In some 

 cases, as in the Oolitic sand at Scarborough, this is apparently due 

 to the removal of what were perhaps grains of shell-sand; but 

 still there is sufficiently good evidence that quartz has been de- 

 posited in situ. In some other specimens, like the New Eed of 

 Penrith, a number of grains may often be seen cohering more 

 strongly than the rest ; and these show clearly that the cavities 

 originally existing between the grains have been more or less com- 

 pletely filled with quartz. Moreover, on carefully examining the 

 less-coherent grains by surface-illumination, we can see, not only 

 the planes and angles due to unimpeded crystallization, but also 

 more or less deep impressions, due to the interference of contiguous 

 grains, thus proving conclusively that the deposition of crystalline 

 quartz took place after the nuclei were deposited as a bed of normal 

 sand. The very imperfect consolidation sometimes met with is, 

 perhaps, not so very surprising when we reflect on the very small 

 coherence of many large quartz crystals which are yet in close 

 juxtaposition. However, it does seem probable that this crystal- 

 lization of quartz sometimes contributes very materially to the 

 cohesion of the grains in hard and compact quartzites. In such 

 examples as the G-annister of the South Yorkshire coal-field we can 

 see, in a thin section, that the grains fit alongside one another in a 

 very striking manner, and it is only by extreme care that good 

 proof can be obtained of the actual deposition of quartz between 



