GEOLOGY IN NATIONAL MUSEUM — MERRILL. 281 



solution, Avhile the term dike refers to intrusions of igneous matter 

 into like preexisting cavities, or fractures, in the older rocks. The 

 distinction between the two is not always apparent inasmuch as it 

 is impossible in all cases to distinguish between a condition of 

 solution and igneous fluidity. The pegmatite intrusions mentioned 

 later are good examples of this feature. Both veins and dikes usu- 

 ally possess length greatly in excess of breadth, and of the two the 

 dikes are the much more important as geological bodies, but of 

 less importance as bearers of interesting minerals and ores. Among 

 the more interesting objects in this collection are the veins of abesti- 

 form serpentine from Thetf ord, Canada ; the banded gold and silver- 

 bearing vein from the Rico district, Colorado; the diabase dikes 

 in granite from Norway, Me. ; and the small filamentous dikes from 

 a boulder at St. Elizabeth, Me., illustrating the extreme fluidity of 

 the magma. 



Contact metamorphism. — The intrusion of a quantity of molten 

 material into older rock masses often brings about a small amount 

 of change, called contact metamorphism. Various phases of this 

 are shown in specimens of altered limestone ejected by the volcano 

 of Vesuvius, and limestone crystallized at contact with a trap dike 

 from Thomaston, Me. 



Metasomatosis. — The form of metamorphism known as metasoma- 

 tosis is illustrated in the series showing the derivation of serpen- 

 tine from a lime-magnesian pyroxene from Montville, N. J. In 

 the large mass shown, the gray core of pyroxene may be observed 

 covered with a thin crust of serpentine and traversed by large and 

 small veins of the same substance, the process having been arrested 

 before completion. In some instances the calcium set free has 

 crystallized out in the form of calcite of a white or blue-gray 

 tint. The exteriors of many of the nodules are often grooved 

 and striated like glacial boulders, owing to expansion and conse- 

 quent crowding in the process of hydration. 



ROCK WEATHERING AND SOIL FORMATION. 



The processes involved in the breaking down of rock masses, 

 and their reduction to the condition of soils, are in part physi- 

 cal, and in part chemical. The physical processes are chiefly those 

 of expansion and contraction caused by heat and cold; the chemi- 

 cal largely those of solution and hydration from atmospheric 

 waters. Other processes involving oxidation and formation of new 

 compounds may go on conjointly, but with the exception of the 

 first named the results are not of such a nature as to lend them- 

 selves readilv to exhibition. 



The following series show the various transition stages from 

 rock to the pulverulent material, ordinarily designated soil: (1) 



