64 PEOCEEDINGS OE THE GEOLOGICAL SOCIETY. 



them. However, in the case of a highly consolidated sandstone 

 from Trinidad the proof of tne deposition of quartz is as complete 

 as possible : the outline of the original grains of sand is perfectly 

 distinct ; and the cavities between them are filled with clear quartz 

 in crystalline continuity with the contiguous grains ; so that the 

 whole is a mass of interfering crystals, each having a sand-gi'ain 

 as a nucleus. The rock has thus been converted into a hard 

 quartzite, almost like a true quartz rock, but differs from such 

 quartz rocks as those of the Scotch Highlands in containing no 

 mica crystallized in situ. All my specimens of these quartz rocks 

 are really highly quartzose mica-schists ; and, so far, I have failed 

 in my endeavours to trace the connexion between them and true 

 sandstones, though possibly this could easily be done in some 

 districts which I have not examined. 



"We can thus trace the history of quartz-sand from its origin in 

 crystalline rocks through different mechanical and chemical changes, 

 all of which may be paralleled with what has occurred in the case 

 of the calcareous sand derived from organic bodies. As shown in 

 my address last year, we have in them a falling to pieces by decay, 

 a mechanical breaking up and wearing down, as well as corrosion 

 and the subsequent deposition of calcite; but of course many 

 of the facts are very different, on account of the totally different 

 chemical properties of carbonate of lime and quartz. 



Internal Structure of the Grains. 



Having now considered the external form of the grains, it 

 remains to consider their internal structure in a few special ex- 

 amples. 



Millstone Grit of the neighbourhood of Sheffield. — The grains of 

 quartz are, on the whole, extremely angular, and, as a general rule, 

 show little trace of wearing ; so that portions of decomposed felspar 

 still frequently fill irregularities on the surface, and greatly inter- 

 fere with our view of the internal structure. A few fragments of 

 undecomposed felspar also occur; but the greater number have 

 been decomposed, and the kaolin squeezed in between the grains 

 of quartz. Occasionally we meet with what may be called grains 

 of granite. On the whole, by far the greater part of the material 

 is as if derived from decomposed granite ; but a few grains are as 

 if derived from schists. The occasional occurrence of pebbles of a 

 somewhat coarse-grained granite, along with many composed 

 entirely of quartz or felspar, strongly confirms the conclusion thus 

 arrived at from the study of the grains of sand. The internal 

 structure of the quartz indicates that the granite was of a type 

 differing from many others ; but one of the most striking characters 

 is that there are many very minute fluid-cavities, with most un- 

 usually minute bubbles, moving about with extreme apparent rapi- 

 dity. Some grains contain flakes of mica ; schorl is rare, if not 

 absent ; but hair-like crystals of r utile are common. 



Blown Sand of the Egyj^tian Desert. — This is an admirably ex- 



