AQUEOUS AGENCIES. 23 



to 3,000 feet deep. All the tributaries which run into the 

 river at this point have cut deep side gorges, headed by 

 perpendicular falls. Some of the most exquisite falls are 

 here nestled among the hills in these almost inaccessible 

 gorges. The country rock is a very hard but much 

 jointed lava, underlaid by a softer cement-gravel. The 

 falls have eaten out the gravel and undermined the lava, 

 which from time to time tumbles into the chasm as blocks 

 that are carried away by the stream. In this Avay the 

 falls have worked back about two miles. 



Yosemite Falls. Most perpendicular falls have been 

 made by recession, as explained above, but this is not true 

 of all. The Yosemite Falls (of which there are six, vary- 

 ing in height from 400 to 1,600 feet) have not perceptibly 

 receded. This is because the granite is very hard, and 

 the time too short (probably only a few thousand years), 

 since the valley was filled with ice (page 394). 



Ravines, Gorges, Canons. These are found in all 

 countries, especially in mountainous and high-plateau re- 

 gions. They are always or nearly always formed by run- 

 ning water, although in some cases their places are deter- 

 mined by fractures of the earth's crust (page 239). They 

 are gullies on a large scale. In the Appalachian Chain 

 the most striking examples are the Hudson River gorge in 

 New York, the Tallulah gorge in Georgia, and the French 

 Broad gorge in North Carolina. But it is in the western 

 part of the continent that the finest examples are seen. 

 Nowhere in the world are they on a grander scale, more 

 evidently due to water alone, or more recent in origin. 

 As we are studying "causes now in operation," they are 

 the most instructive examples to be found anywhere. 



In California there was, even since middle geological 

 times, an old river-system different from the present. 

 This will be explained more fully hereafter (page 395). 

 These old river-valleys were filled up with river-gravel, 

 and finally obliterated by lava-flows not long before the 



