268 /. P. GOODE 



pushing its growing gorge back into the rhyolite of the plateau. 



In Fig. 3 the site of the future Grand Canyon is represented, 

 the contours being copied from the U. S. Geological Survey topo- 

 graphic map, with the exception that south from the mouth of 

 the present Broad Creek the contours of Broad Creek itself are 

 supplied, in the line of drainage of Sulfur Creek. The col 

 between the Sulfur Creek gorge and the greater lake lay about 

 two miles north of Inspiration Point in the old Continental 

 divide. Yet the Sulfur Creek pirate was a long time eating 

 thru this two miles or so of barrier. And all this while — a 

 good fraction of postglacial time — the great lake was giving 

 its water thru the Overlook Mountain channel to the Snake 

 River, and the beaches, terraces, and sea cliffs were building at 

 the contour of 7900 feet — about 160 feet above the present lake 

 level. In the Hayden Valley part of the lake, similar beach 

 records were making, and the stratified clays were being deposited 

 off-shore. 



The rate of advance thru the col by the Sulfur Creek 

 pirate would depend upon three factors, the volume and gradient 

 of the stream, and the nature of the rhyolite. The volume of 

 water was not large, being only the drainage from the south 

 flank of Mt. Washburne and the east flank of Dunraven Peak. 

 The gradient was high, about 1500 feet, in the Sulfur Creek 

 branch alone, while the rhyolite in the path of the canyon was 

 in admirable condition for easy working. 



The rhyolite, on first cooling from its flow, was hard and firm 

 of texture, the obsidian or volcanic glass being one phase of it, 

 usually found at the surface. In deeper levels it may have been 

 as hard and crystalline as basalt ; but the hot vapors from below 

 have attacked the firm rock and in many places totally changed 

 its character, making the felspars over into kaolin and leaving 

 the once firm lava a crumbling mass, almost like slaked lime. Yet 

 this solfataric action has not been universal. It has worked very 

 effectively in certain areas, while in other places the solid rhyolite 

 has wholly escaped the decomposing action. Were this not so, the 

 canyon would long ago have advanced clear to the present lake. 



