20 Dr. W. F. Hume — On Walther's Desert Erosion. 



He concludes that the 'ripples' or 'vermicular' markings are 

 produced in the soil and near tlie surface through the action of 

 concentrated solutions rising to the ground-level. As they slowly 

 move along the side of the limestone fragments they produce linear 

 markings. The removal of the clayey material reveals the etched 

 surfaces, which are gradually worn down by the sand, while new 

 ripple-stones are uncovered as the clay is removed by wind from 

 around tliem. 



There is always an advantage in having some explanation given 

 for any phenomenon, because it at once arouses the critical faculty 

 in the majority of mankind, and this tends to progress. Like 

 Professor Walther, I have sought for an explanation as a solution 

 effect, in which dew or rain, concentrated solutions of contained 

 salts, and u particular rock-structure resulted in these curious 

 markings. 



Professor Walther is naturally also impressed with the changes 

 on rock masses themselves resulting from the movement of highly 

 concentrated solutions through them. In this connexion reference 

 is made to the remarkable grape-like aggregations which are formed 

 when finely divided carbonate of lime, clay, and quartz sand are 

 moistened and subsequently become bound together under the 

 infl.uence of the sun's heat. He next turns to a question which has 

 aroused much interest, viz. the origin of the gypsum present in 

 enormous thickness near the Red Sea and elsewhere. Attention is 

 called to Professor batterer's observation that the waters of the Red 

 Sea penetrate for long distances into the rocks of the shore-line, and 

 there produce metasomatic changes. It is rightly pointed out that 

 these regions are like a huge retort, in which remarkable chemical 

 changes may be proceeding on a vast scale. Professor Walther 

 frankly adopts the view that the gypsum is not a primary deposit, 

 but represents the wall of the Erythraean depression, formed originally 

 of marine limestones, but subsequently altered to gvpsum. 



In this connexion he calls attention to the pseudomorphs in selenite 

 of bivalves, gasteropods, and corals collected by Dunn on an island 

 south of Berenice. Far from this being an isolated case, examples 

 are available from many parts of Egypt. At Abu Mingar there is 

 a very interesting stratum in which a very inflated bivalve {Biplo- 

 donta) is formed of glassy-looking selenite ; at Jemsa a bed of small 

 Corhulce is entirely replaced in gypsum, and the change can also be 

 observed taking place in a limestone bed on Little Jemsa. The 

 surface layers of calcareous strata are frequently altered to powdery 

 gypsum, and Wade has called attention to the metamorphosis of the 

 large basin shell Tridacna into the same material. 



iNor is this the onh' case of formation of sulphates. The Survev 

 Chemical Laboratory has examined many specimens collected by the 

 Geological Survey, the analysis of which show that sulphates of 

 alumina and iron play a most important role in desert deposits. 



The change of ordinary limestone to a true dolomite by the action 

 of magnesian salts is equally impressive. Sea-water breaking 

 through the open-work structure of projecting masses of coral show 

 replacement of this kind to a marked degree. The rapidity of 



