INTRODUCTION. 97 



had been adopted by his predecessors. Instead of confining 

 himself to a description of the superficial aspect of the volcanic 

 mountains and the characteristic phenomena of eruption, Dolo- 

 mieu studied the lavas, loose ejecta, sublimations, etc., and 

 compared these volcanic products with other rocks. He thus 

 arrived at the result that all transitional stages exist between 

 the coarsely crystalline lavas and the glassy rocks (obsidian, 

 pitchstone), the latter being merely particular structural varieties 

 of the crystalline lavas. 



In order to explain the possibility of so many grades of 

 structure, Dolomieu supposed that volcanic heat, unlike any 

 kind of artificial heat that could be produced in the laboratory, 

 did not reduce the original rock-material to a completely melted 

 mass, but merely to a viscous state, in which the individual 

 mineral constituents could move relatively to one another 

 while still retaining their characteristic form. 



He further supposed the lavas contained a combustible sub- 

 stance (perhaps sulphur), which held the rock in this viscous 

 state until it was completely consumed ; and that this com- 

 bustible substance, by its expansive force, produced the 

 scoriaceous, slaggy, and irregular surfaces of lava streams, 

 and caused the upward pressure of molten magma to the 

 orifice of escape. 



Dolomieu confirmed the igneous origin of basalt rock, re- 

 garding it as a variety of lava for the most part associated with 

 submarine eruptions. He compared the alternating lava streams 

 and sedimentary strata at Etna with the stratigraphical relations 

 of the so-called trap-rocks in the Vicentine district, and con- 

 cluded that the latter gave evidence of volcanic activity. 



The name of Dolomieu is perpetuated in the name of the 

 " Dolomites," given to the beautiful district in South Tyrol 

 south of the Puster Valley. Dolomieu called attention in 1791 

 to the unusual mineralogical character of the " Alpine lime- 

 stone" in that district. His chemical investigations proved the 

 rock to contain, in addition to lime carbonate, a very high per- 

 centage of magnesium carbonate ; so that the rock could by no 

 means be regarded as a true limestone. Afterwards, any highly 

 magnesic limestone came to be called "Dolomite" rock. 



In 1797 Dolomieu confirmed the statement of Giraud 

 Soulavie, that the volcanoes of Auvergne and Vivarais are 

 intruded into the granite, and partially rest upon it. Thus 

 Dolomieu extended our knowledge of the mineralogical com- 



7 



