1886.] NEAV YORK ACADEMY OF SCIENCES. 120 



of great extent. It is said that the fissure of the Smuggler 

 vein can be traced for two miles. 



Proving that the trachyte was erupted at different ages, we 

 find veins formed and filled in one age by one peculiar kind of 

 filling, and \covered over in their course by another kind of 

 trachyte without any veins whatever. 



San Juan veins have produced in the last year one-fourth of 

 all the silver of Colorado, 



The geological order of events in the construction of these 

 mountains seems easy to read. I speak with diffidence, having 

 not seen actual proof, but I think that the Needles and other 

 elevations at the head of the Rio Grande are much the oldest 

 mountains. At the close of the cretaceous age they were sur- 

 rounded by a vast plain — part of the great mesa country of S. A¥. 

 Colorado. 



Forces operating from the earth's interior broke through sedi- 

 mentary strata and forced up masses of metamorphic roc,k. In- 

 ferior Silurian, carboniferous, and cretaceous rocks all felt the 

 elevating influence, and were tilted up — turned on edge — or 

 lifted solid and unbroken. They presented rugged heights and 

 deep, narrow gorges. Then followed an age of eruption of thick, 

 tough, pasty material which filled the valleys, covered the rugged 

 heights, and every remnant of mesa country that was caught in 

 its grip. This molten matter was forced through every cranny 

 and crevice of rocks. Sometimes it lifted up immense niasses of 

 horizontal strata by what is now called laccolite elevation. It 

 seems to have had the power of changing all other rocks it came 

 in contact with into something of its own characteristics; for by 

 heat, steam, and j^ressure, and the interplay of chemical reac- 

 tions, the original features of the stratified rocks have been com- 

 pletely destroyed. 



It is only at distant intervals that we find any rocks which give 

 a clue to their origiiuil condition. In Denver Hill we find an 

 altered slate whicli we can identify with the black slates at Bear 

 Creek Falls. At the foot of Engineer Mt., in the upper canon, 

 we find red sandstone slabs not yet fully changed. 



This first period of outflow might be called by way of distinc- 

 tion, the porphyry 2^eriod. 



Then followed "another out-push of melted rock. It came up 

 through small craters, chimneys, and vent-holes. There were 

 many thousands of such vents; each peak had its own set and 

 series of them. The hot material was viscid and tough. While 

 it would not flow, it could be pushed along by a vis a tergo, and 

 could be piled up. It could form domes and cones on the top of 

 every mountain, but it was not sufficiently fluid or semi-fluid to 

 flow down the mountain side and fill up the valleys. 



