THE DESERT SANDSTONE. 313 



to say what these may be, have made some sort of a classification 

 by arranging them under the heads of microliths, crystalloids, 

 trichites, and globulites. Microliths are imperfectly developed 

 crystals, often possessing optical characters which enable their 

 nature to be determined. In sections of certain volcanic rocks, 

 streams of microliths with their longest axis in one direction may 

 be seen sweeping in curves round the larger crystals and fragments. 

 Crystalloids manifest a higher development, being bounded by 

 curved or straight lines, and sometimes stellate and cruciform 

 varieties ; often too in the form of true crystals which can be 

 recognised. Trichites are like hairs or fibres, more or less straight, 

 curved, or bent in all kinds of angles and twists, twirling in the 

 most fantastic modes round larger granules. Trichites often are 

 lines of granules like beads in rows or in pairs. Finally the name 

 globulites is reserved for those amorphous and roughly spherical 

 bodies which cannot be identified with any of the other categories; 

 though these shapeless masses are symmetrical often in their mode 

 of grouping, and are also arranged in streams in the viscid lavas. 



Now when volcanic sands are very fresh, we find all the above 

 inclusions well represented and unmistakably present ; but I 

 regret to add that it does not take a very long time to destroy 

 them. Chemical interchange goes on, oxidation and crystallisation 

 accompanied with the weathering action of water, so as to obliterate 

 most of the former characters. I wish I were able to say after 

 having spent so much time in the microscopic examination of sands, 

 that I have discovered any definite mark or character by which 

 the history of the mere quartzose residuum could be determined, 

 that is to say the nature of its former genesis; but I repeat to my 

 regret that such evidence is not always very visible. It is true 

 that even when the stone is apparently an aggregation of pure 

 siliceous grains, there are always some foreign minerals left which 

 may help to determine its origin ; but it must be admitted that 

 the evidence is not always of a conclusive or satisfactory kind. 

 Without wishing to rely upon such facts for more than they may 

 be worth I will here notice some that have fallen under my 

 observation, which may help to throw a light upon the origin of 

 these Desert Sandstones. 



First of all is the shape of the grains which are rounded, and 

 this apparently not from attrition. Eolian sands usually are 

 rounded ; but they are also often opaque. Some of the sands are 

 rounded and egg-shaped and have a decidedly molten look about 

 them, such as I have seen in volcanic glass ; but this is not a 

 universal character. Some of the Desert Sandstone has angular 

 grains though roughly spherical in shape. Partial crystallisation 

 has taken place amongst the grains in many instances, and this 

 prevents the former figure from being now discernible. 



