28 SC/£AT£ PROGRESS. 



Iddings interprets this wider range of variety in the northern 

 area as indicating that evolution is there in a more advanced 

 stage than in the south. The province of the Andes, with 

 its still active volcanoes, is in its youth ; the Great Basin, 

 where vulcanicity seems to be practically extinct, has 

 reached middle or old age. Even in the latter case, how- 

 ever, the rocks evince no very high grade of specialisation, 

 and the same types recur in different parts of the province. 

 A strong contrast in these respects is presented by the 

 rocks of the alkali group on the eastern slope of the "Great 

 Divide," where, as Iddings has shown, both specialisation 

 and localisation are carried to a high pitch. 



A similar contrast is seen on comparing the intrusive 

 rocks developed in the same provinces. It must be re- 

 membered that the earlier identifications of the igneous 

 rocks of the Western states often require revision. Hague 

 and Iddings have pointed out that many of the lavas named 

 trachyte by Zirkel in Nevada and elsewhere are more 

 correctly designated hornblende-pyroxene-andesite, and it 

 does not appear that true trachytes have been identified 

 west of the Rocky Mountains. The intrusive rocks which 

 build the well-known laccolites of the Henry Mountains in 

 Utah and similar masses in the West Elk, Mosquito. Abajo, 

 La Sal, and other mountain groups of the high plateaux 

 country have also been described in various places as 

 trachytes. Whitman Cross in a recent memoir has shown 

 that they have no right to this title, but are in general 

 horneblende porphrites (6). A very interesting point brought 

 out by the author is the remarkable uniformity exhibited by 

 the rocks of these great laccolitic intrusions over a tract 

 extending from New Mexico northward through Utah and 

 Colorado, and apparently at least as far as the borders of the 

 Yellowstone Park. To this persistence of comparatively 

 simple types of porphyrite and diorite throughout the High 

 Plateaux and Great Basin tract are strikingly opposed the 

 curious intrusive masses and complexes of Montana and 

 other areas within and on the eastern edge of the Rocky 

 Mountain belt, comprising augite and sodalite-syenites and 

 theralites, with such peculiar modifications as the yogoite 



