266 W. LINDGREN TWO NEOCENE RIVERS OF CALIFORNIA. 



of the rivers then meandered over floodplains two or three miles wide, 

 above which the divides of bed-rock rise to a height of several hundred 

 feet. In some instances low passes over divides were covered, and tempor- 

 rary bifurcation and diversion of rivers into adjoining watersheds occurre< I . 

 The volcanic Period. — At this time the first eruptions of rhyolite began 

 from the first summit, from the volcanic center of Castle peak, they 

 poured down the valleys, at first as molten flows, then as fine breccias 

 and tuffs, and, being mixed with detrital material on their way, they arc 

 finally found lower down as semi-volcanic sands, clays and gravels. 

 These beds of mixed volcanic and sedimentary character, usually fine- 

 grained and distinguished by a brilliant white color, have been collectively 

 distinguished under the name of rhyolitic beds, and merge into the tuffs 

 and massive rhyolites of the upper valleys. The rhyolitic flows usually 

 confined themselves to the valleys and only in some instances flooded 

 certain of the low passes. The maximum thickness of these flows near 

 the summit is one thousand feet, but it rapidly diminishes westward. 

 These masses of fine detritus flooded the lower slopes and compelled the 

 rivers to seek new channels, still, however, in general confined to the old 

 valleys. The waters at once began the work of cutting down in the 

 clave}- and sandy masses. Then the period of the andesitic eruptions 

 began, Dark-colored mud flows, at first sandy and clayey, again flowed 

 down the valleys ; the divides began to be covered. Again the rivers 

 were displaced and again they at once began their work of active erosion, 

 cutting down not only through the accumulated silt to their former 

 levels, but. wherever the intervals between the eruptions allowed it, down 

 through the gravels sometimes deep into the underlying 1ied-n»ek. In 

 some districts, especially on the American river, these intervolcanic 

 channels cut and destroyed again and again the older deposits, pursuing 

 a wholly independent course, although in general flowing in the same 

 valleys. They have exactly the same characteristics as the modern 

 rivers. It is important to note that they enable us to fix with accuracy 

 the relative date of the change from the conditions of the Neocene to the 

 conditions of to-day. Whatever causes produced this change, they 

 began to act at this time. The intervolcanic channels were of an ephem- 

 eral character; successive eruptive flows changed their direction, and it 

 is to be expected that there were, as shown by Mr Browne, several dif- 

 ferent systems of them. They occur principally in the watersheds of the 

 American river where the interval between the first and the lasl andesitic 

 flows seems to have been considerably larger than in the region of the 

 Yuba. Cement channels, as the intervolcanic channels are often called. 

 no doubt also occur in the latter, but as a rule they did not have time to 

 cut down far into the rhyolitic beds before they were Idled and replaced 



