AENIYEESAEY ADDKESS OF THE PEESIDENT. 47 



igin of Material. 



Though the material of any more or less modern non-calcareous 

 stratified deposit may have been derived from some analogous rock 

 of earlier date, yet we must, I think, trace back the greater part to 

 the mechanical breaking-up or chemical decomposition of igneous 

 or metamorphic rocks, since in them by far the greater bulk of 

 new crystalline material is formed. I therefore purpose now to 

 consider the character of the sand and mud thus derived from 

 different classes of rock. 



The minerals of truly igneous rocks have been formed at an 

 elevated temperature, and free from the continued solvent action 

 of water. Though some are very stable, and do not undergo any 

 material change when exposed to water at the ordinary temperature, 

 yet many are altogether in a state of unstable equilibrium when 

 exposed continuously to conditions so extremely different from those 

 under which they were formed. Some of the constituents are dis- 

 solved out ; and the rest group themselves in accordance with the 

 new affinities, which vary according to special circumstances. Even 

 in the case of those plutonic or metamorphic rocks which have 

 been formed more or less under the influence of water, the tempe- 

 rature was apparently so elevated that the mutual affinities of the 

 constituents were unlike those brought into play by exposure to 

 the continued action of cold water, especially if it is charged with 

 carbonic acid. When igneous or metamorphic rocks are weathered 

 and broken up, we thus obtain sand and fine mud composed not 

 only of the original materials, but also of products of chemical 

 change. I shall therefore now consider some of the more im- 

 portant constituents of stratified deposits derived from certain 

 typical rocks. 



Quartz Sand. 



The quartz of granitic rocks sometimes shows more or less im- 

 perfect crystalline planes, but is more commonly dovetailed in 

 amongst the other minerals in a very intricate manner. The result 

 is that in the rock itself the separate grains have often a most 

 irregular and complex form, but yet on the whole are not much 

 drawn out or flattened in any special direction. When the felspar 

 is decomposed, and the rock broken up, some of the occasional 

 special inequalities in the length, breadth, and thickness of the 

 grains of quartz are reduced by cross fractures ; and the resulting 

 fragments are usually not more than, at all events, twice as long 

 in any one direction as in any other, and have a very irregular im- 

 perfectly angular or imperfectly rounded outline. The quartz in 

 quartzose felsites is often of much more truly crystalline form, 

 the planes being sometimes very perfect ; but very often there is a 

 remarkable rounding of the angles, which might easily lead any one 

 to think that they were waterworn. Even the grains of quartz 

 derived from granite sometimes show this character to a less ex- 

 tent, but the rounding is usually accompanied by small surface- 



