ioO TRANSACTIONS OF THE [jAN. 25, 



This period I should call the tracliytic. 



On cooling, these outflows hecame fractured and fissured. 

 These fissures became the heated drains of mineral matter, and 

 were charged with precious metals, in siliceous gangues, in the 

 tertiary age. 



The evidence of this age I have not seen, but take it as 

 proven by Worthen's surveys. Certain it is, they are now the 

 natural vaults of King Pluto's wealth. 



DISCUSSION. 



President Newberry said that he had spent seven months 

 in this region in the year 1859, while connected with the San 

 Juan Exploring Expedition, and had described some of its geo- 

 logic features in his report to Capt. J. N. Macomb, U. S. 

 Engineers. 



Dr. Stevens has not exaggerated the pigturesqueness of the 

 region nor its mineral wealth. There is no part of the continent 

 where the alpine scenery is bolder and more varied. The Rocky 

 Mt. belt, here two hundred and fifty miles wide, consists of a 

 series of ranges which terminate southerly in echelon — the San 

 Miguel range first, and in succession the Sierra la Plata, Sierra 

 de los Pinos, San Juan, and others. These end abruptly, giving 

 place on the south to a broad plain traversed by the San Juan 

 River and its branches. 



The mountains are well watered, snow falling heavily in 

 winter, and showers occuring almost every day in summer, while 

 the plain near by is extremely dry. The latter, however, was 

 once inhabited by a dense popuhition, the Mexican semi-civil- 

 ized people, who cultivated the soil and built stone houses, fre- 

 quently of great size, and in such numbers that their ruins are 

 always in sight of the traveller. 



The President exhibited specimens of ore from the Madonna 

 mine. Monarch, Colorado, in which the galena had changed to 

 sulphate of lead by oxidation; and by the action of water the 

 latter was given a partially concretionary structure. 



