proceedings: geological society 45 



The glaciers and glacial sculpture of the Park. F. E. Matthes. Both 

 papers were illustrated. Edson S. Bastin, Secretary. 



The 242nd meeting was held in the Cosmos Club, March 22, 1911. 



REGULAR PROGRAM «• 



The stability ranges of minerals with special reference to the geologic 

 thermometer scale. Fred E. Wright. 



The physico-chemical systems represented by rocks and the geologic 

 forces which act upon these systems are often so complex that it is not 

 possible at present to solve completely the problems which they present 

 and to state definitely the behaviour of any particular rock or magma 

 under the different conditions of temperature and pressure which may 

 arise. It is important, therefore, that limits of temperature and pres- 

 sure be established, beyond which any given system or any component 

 of the system is no longer stable but undergoes change. These limits 

 mark points on the geologic thermometer scale or pressure gauge and 

 place definite boundaries to the region in which the observed reactions 

 have taken place. At these points an abrupt shift in the equilibrium 

 of the system occurs and produces profound changes in the system. 



The most important thermometric points which may be of service 

 in this connection are: (a) Enantiotropic inversion temperatures of 

 minerals, as that of a quartz into /3 quartz; (b) temperature limits 

 beyond which monotropic or isomeric forms cannot exist, as the tempera- 

 ture 450°, above which marcasite passes at atmospheric pressure into 

 pynte* (c) stability ranges of enantiotropic forms and of minerals which 

 dissociate or decompose at higher temperatures, as the dissociation tem- 

 peratures, for different pressures, of calcite and other carbonates or of 

 certain of the garnets and many other contact minerals; (d) melting tem- 

 peratures of stable minerals and of eutectics : (e) temperatures beyond 

 which any physical property acquires a permanent set and by virtue of 

 internal friction or other cause does not revert to its original state on 

 cooling. In applying these data of laboratory experiment to geologic 

 problems, it is important to note the conditions under which the data 

 were obtained and the effect which other conditions of pressure or solu- 

 tion may have exerted on the observed points of equilibrium change. 



Physiographic studies on the south slope of the San Juan Mountains, 

 Colo. Wallace W. Atwood. 



The glacial features on the south slope of the San Juan range do not 

 differ from those which have been described by various writers in the 

 high mountains of the West. Moraines of two epochs have been recog- 

 nized and outwash deposits are associated with each. On ridges above 

 the limit of ice action are certain gravel-strewn surfaces, which, if extended, 

 would form a plain with gently rolling topography. This plain, sloping 

 away from the range, would show distinct down warping at its base, 

 and merge with the upland surfaces of the neighboring plateaus. These 

 gravel-strewn upland surfaces are probably portions of a deformed pene- 

 plain. Beyond the mountains in the plateau district the peneplain 



