16 REPORT OF NATIONAL MUSEUM, 1889. 



that the pebble is today apparently as strong as ever. Still other 

 effects of pressure and shearing are shown in the broken trap-dikes and 

 veins of serpentine in the crystalline limestone of Smithfield, Khode 

 Island (69560-69572), and the foliated structure of certain gneisses. 



The phenomena of jointing, produced by a sharp shock or an abrupt 

 fold, is illustrated on a small scale by a specimen of gneiss from Cape 

 Elizabeth, Maine, in which the rock is divided at intervals of a few 

 inches by rifts as sharp as though made by a lapidary's wheel. Other 

 peculiar forms of jointing are shown in shale from Cambridge, Massa- 

 chusetts (70590), and slates from Carlton County, Minnesota (26488). 

 Joints in igneous rocks and other specimens illustrative of the phe- 

 nomena on a larger scale may be found under the structural series. 

 (See p. 28.) 



The movement of a mass of rock along the line of a joint gives rise 

 to what is known technically as a fault. Specimens illustrative of this 

 feature and suitable for exhibition purposes are as a rule difficult to ob- 

 tain owing to their size. Nevertheless it occasionally happens that 

 good examples may be had on a scale sufficiently small for exhibition. 

 In each of the cases exhibited the amount of displacement was slight, 

 and is readily determined by the bands of various color by which they 

 are traversed. In these instances, as is not always the case in larger 

 rock masses, the faulted blocks have become reunited so firmly that the 

 fault would scarcely be suspected but for the fact that the color bands 

 are no longer continuous. (Specimens 72869 from Montana and 20809 

 from Nevada.) 



The slipping of one mass of rock over another along a line of fault 

 give rise to smooth and striated, often highly polished surfaces known 

 as slickensides. These are shown on massive magnetite from New 

 York State (37039) ; anthracite coal from Pennsylvania (70660) ; and 

 the wall rock of silver mines in the Keese Eiver district Nevada (31638). 



(3) The metamorphism of rocks. — The subjects of the metamorphism 

 of rocks by the heat of injected volcanic masses has already been 

 touched upon (p. 14), as has also that form of change produced by move- 

 ments in the earth's crust and resulting in a schistose foliated or brec- 

 ciated structure (p. 15). There is one other form of change, however, 

 which can perhaps be best illustrated here. This is the change known 

 as mgtasomatosis, a process of indefinite substitution and replacement. 

 The details of this process are admirably shown in the series illustrat- 

 ing the origin of serpeutinous rocks. 



Serpentine, it should be remembered, is essentially a hydrous silicate 

 of magnesia, consisting, when pure, of nearly equal proportions of 

 silica and magnesia with from 12 to 13 per cent, of water. The massive 

 varieties occurring in nature are, however, always more or less impure, 

 containing frequently from 10 to 12 per cent, of iron oxides, together 

 with varying quantities of chrome iron (chromite), iron pyrites, horn- 

 blende, olivine, minerals of the pyroxene group, and the carbonates of 

 lime and magnesia. 



