122 ON EROSIONS OF THE EARTH'S SURFACE. 



14. Source of the River Jordan, above Lake Hulefi, in the mountains of Lebanon. — 

 Mr. Thompson, American missionary, describes it as a constantly deepening gorge, 

 in basalt, six miles long. — Bibliotheca Sacra, vol. III. p. 135. 



15. Valleys in the volcanic islands of the Pacific Ocean. — These valleys have been 

 described, and their origin discussed with great ability, by Professor J. D. Dana, in 

 his Geological Report of the United States Exploring Expedition. He divides 

 them into three kinds: 1. " A narrow gorge with barely a pathway for a streamlet 

 at bottom, the enclosing sides diverging upwards at an angle of 30° to 60°." 2. 

 "A narrow gorge, having the walls vertical, or nearly so, and a flat strip of land 

 at bottom, more or less uneven, with a streamlet." 3. "Valleys of the third kind 

 have an extensive plain at bottom, quite unlike the strip of land just described." 

 The valleys are one, two, or even three thousand feet deep, and the dividing ridges 

 often so narrow as to be knife-like and tortuous. Professor Dana imputes their 

 origin mainly to two causes : first, volcanic agency, which lifted up the mountains 

 and produced inequalities and gulfs. Secondly, the action of rains producing 

 brooks and rivers. The latter cause he thinks the chief one, though the ocean, 

 especially when the islands were nearly submerged, must have produced some 

 effect. 



The phenomena of valleys in some parts of the great Appalachian coal field, as 

 along the Ohio and Great Kenawha rivers, appear to me to sustain the view taken 

 by Professor Dana, that streams of water chiefly have eroded the valleys of the 

 Pacific islands. For along those rivers the coal measures lie nearly horizontal, 

 and the rivers have evidently worn out their beds to the depth of some hundred 

 feet, leaving bluffs of sandstone along their margin. And wherever brooks and 

 streamlets have found their way to the main river we observe that they have worn 

 out channels having the same steep sides as those of the Pacific islands. The 

 ridges too, intervening between the brooks, are sharp, narrow, and tortuous, though 

 not extremely so, like those of the Pacific islands. Now, in the horizontal coal 

 strata, which have never been disturbed, we can impute the valleys and ridges to 

 nothing but running water, and it is reasonable to refer the analogous phenomena 

 of the Pacific islands to the same cause. 



Conclusions. 



From the facts that have been detailed, we may derive several inferences of con- 

 siderable geological importance. With these I shall conclude this paper. 



1. Some of the erosions that have been described, must have been commenced 

 as early as the oldest rocks were consolidated. 



They occur in the oldest hypozoic rocks, and were begun probably by the drain- 

 age of land at its first emergence from the waters. The hypozoic rocks are not 

 indeed necessarily older than the fossiliferous. But sometimes they lie below the 

 fossiliferous, and are too thick to be regarded as their lower metamorphic beds. I 

 should place the following cases as among the earliest described in this paper : — 



1. Valley of Connecticut river, which is for the most part formed in hypozoic 

 rocks. 



