4 PRESIDENT S ADDRESS. 



solutions. The solid rocks of the land are broken up by changes 

 of temperature, by friction of one piece against another when 

 moved by gravity, wind or water, and by animals and plants. 

 The rounding-off of the edges of the grains as derived from the 

 breaking up of the parent rock is much more complete when 

 done in the air than in water, and the process is carried on 

 in smaller particles ; the limit of size of a rounded water- worn 

 grain of quartz has been found by experiment'' to be about O'l 

 mm., but much smaller particles have their edges worn off when 

 blown about with other grains of sand. With the very important 

 exception of quartz, almost all the common rock-forming minerals 

 are chemically altered near the surface of the ground b}' water 

 containing in solution various substances derived from the air 

 and from organisms; and this water dissolves an appreciable 

 quantity of them, even of quartz. It is in the character and 

 extent of this alteration that the influence of climate makes itself 

 felt, chiefly through its control of hfe^. A very dry climate is 

 unfavourable to the existence of animals and plants, and the 

 mineral grains in such a climate are mainly the result of the 

 physical disintegration of the rock; the felspars, for instance, 

 the most important of the primary rock-forming minerals, retain 

 in such a climate the optical characters peculiar to them and can 

 be recognised in the smallest particles resolvable by the micro- 

 scope. So when we find beds of sediment in which there is much 

 felspar in small grains, there is a presumption that the land which 

 furnished the grains had a dry climate. There is probably no 

 country so dry that the silicates are not partly decomposed, but 

 owing to the amount of water being insufficient to gather into 

 streams or to leak away through joints, much of it evaporates at 

 or near the surface and substances held in solution are deposited 

 amongst the particles forming the soil. Carbonates of lime and 

 magnesia, hydrated oxides of iron, silica and sulphate of lime 

 and chloride of sodium are the chief substances deposited in this 

 way in dry climates. 



Moist, warm countries have deep soils made of clay and quartz 

 sand; the abundant vegetation in such countries, from bacteria, 

 low in the scale of life but probably the most important of all 

 organisms in bringing about changes in minerals, to the forest tree, 

 co-operates with animal life in destroying the original characters 

 of rocks and minerals, leaving behind clays, silica and soluble 

 salts. A product confined to tropical climates, and apparently to 

 regions where there is a marked contrast between wet and dry 

 seasons, is laterite, a residual clay rich in hydroxides of alumina 

 and iron ; somewhat similar rocks without free alumina are formed 

 in temperate climates, both at and below the surface. 



There is no sharp division between arid and humid climates, 

 and only the exti*eme types, arid on the one hand, and wet and 

 hot on the other, impress readily recognisable characters upon 

 the contemporaneously formed sediments. 



The effect of a very cold climate upon its sediments resembles 

 that of arid conditions in that it also restrains vegetation, so that 



