PRELIMINARY HANDBOOK OF THE DEPARTMENT OF GEOLOGY. 21 



water miming down tbe face of the cliff (703§3), and by blocks of gyp- 

 sum eroded by similar means (35590). 



Many rocks show the effects of solution unequally, owing to the un- 

 equal hardness and solubility of their various parts. Thus the lime- 

 stone from Bear Paw Mountain, Montana (28724), is strongly ribbed by 

 siliceous veins from between which the calcium carbonate has been dis- 

 solved. Similar phenomena are shown in specimen No. 37035. 



The formation of carbonates is carried on in such a way as not readily 

 to be shown in a series of this kind. The specimen exhibited is an 

 eruptive rock in which many of the silicate minerals have undergone 

 decomposition, giviug rise to an abundance of carbonate of lime or cal- 

 cite. 



Hydration by itself can be best illustrated in the conversion of olivine 

 into serpentine (see p. 10), and is also shown in the derivation of gypsum 

 from anhydrite, but the change in appearance is, in the latter case, 

 scarcely sufficiently marked to be appreciated. Hydration accom- 

 panied by oxidation, whereby the entire mass of substance falls to frag- 

 ments, is well shown in the py rite-bearing coals of Virginia, and hydra- 

 tion accompanied by the production of carbonates by the pyroxene 

 altered into serpentine. 



The general destructive effects of weathering are shown in an inter- 

 esting series of fresh and decomposed grauitic rocks from the District 

 of Columbia, in which may be traced all gradations from the compact 

 fresh rock through specimens more or less kaolinized and oxidized to 

 soft pulverulent material, upon which plants may be grown. The prin- 

 cipal changes that have taken place being, aside from a physical disin- 

 tegration, an assumption of water, a removal of the alkalies potash 

 and soda, and a conversion of the combined iron oxides into free hydrous 

 sesquioxides, whereby the rock has changed from a gray to a bright 

 umber red. Other objects of like nature here displayed are residual 

 clays from the Southern States, as described by Mr. I. 0. Russell in 

 Bulletin No. 51 of theU. S. Geological Survey.* The exhibit is accom- 

 panied by a few specimens and photographs, showing spheroidal and 

 other types of weathering common to various rocks. 



Water percolating through the superficial portions of the earth's 

 crust dissolves certain constituents either directly or sets up a series of 

 chemical changes resulting in the production of soluble compounds 

 which are gradually removed to be deposited elsewhere or perhaps 

 carried down into the ocean. Nearly all spring waters are hard, owing 

 to the amount of mineral matter contained by them, while rain waters 

 are soft owingto the absence of all mineral matter. By the dissolving 

 power of water are formed the numerous caverns so common in lime- 

 stone regions. By the deposition of mineral matter held in solution 

 are formed a variety of products, some of which are very beautiful. 



* Ou the Snbaerial Decay of Rocks and tbe Origin of the Red Color of Certain 

 Formation. 



