The deposition of material held in solution is most notable at two zones, one below that of most active solution, and the other at the surface, where evaporation is active. Under proper conditions, however, deposition may take place at any level reached by water. _Mechanical Work._ The mechanical work of ground-water is relatively unimportant. Wherever it is organized into definite streams, the channels through which it flows are likely to be increased by mechanical erosion as well as by solution. Either beneath the surface, or after the streams issue, the mechanical sediment carried will be deposited. [Illustration: +Fig.+ 202.—Diagram to illustrate the general form and relations of caves developed by solution. The black portions represent the cavern spaces. Some limestone sinks are represented on the surface above.] RESULTS OF THE WORK OF GROUND-WATER. [Illustration: +Fig.+ 203.—Ground-plan of Wyandotte Cave. The unshaded areas represent the passageways. (21st Ann. Rept., Ind. Geol. Surv.)] =Weathering.=—Where the solution effected by ground-water in any locality is slight and equally distributed, the result is to make the rock porous. If, for example, some of the cement of sandstone is dissolved, the texture of the rock becomes more open; but if all the cement be removed the rock is changed from sandstone to sand. If a complex crystalline rock contains among its many minerals some one which is more soluble than the others, that one may be dissolved. This has the effect of breaking up the rock, since each mineral acts as a binder for the rest. It might happen that no one of the minerals is dissolved completely, but that some one of them is decomposed by water, and certain of its constituents removed. Such change would be likely to cause the mineral so affected to crumble, and with its crumbling, if it be an important constituent of the rock, the integrity of the rock is destroyed. Where considerable chemical changes, especially subtractions, are going on, the rock is likely to crumble. The increase in volume attendant on hydration, etc., sometimes leads to the disruption of rock. These are phases of weathering. (For other phases of weathering see pp. 54 and 110.)