266 The American Oeologist- May, isoo 
feet below their original level. This process of melting away 
of the clay beneath the lava is well shown along the basalt 
front where the lava has faulted olEf in long blocks in front of 
and below one another in a series of steps, which, at first sight 
seem to indicate a series of distinct flows. 
In this vicinity there are many places where the surface is 
not strewn with basalt boulders although on nearly every side 
the boulders or basalt capping is found. At present such 
places are valleys but they undoubtedly represent the hills of 
the time when the basalt flowed over this country. This is 
very strikingly shown on the east side of the Rio Grande oppo- 
site the railway station of Embudo. At this point there is a 
high caiion wall of lava-capped sand , but just behind this, at 
a distance of only a few miles, there is a broad and rather deep 
valley. At the time when the lava reached this point sixty 
miles south of the cone the present valley was undoubtedly 
high land which the lava was unable to cover, and hence un- 
protected by lava it has readily fallen to a lower level than the 
neighboring valley of early times. 
The Rio Grande on account of this rejuvenation is a super- 
imposed river. To what extent its present course coincides 
with the course of early youth is difficult to say ; but, in the 
northern portion, which I have studied particularly, I am in- 
clined to believe that the rejuvenated course and the ancient 
course are practically the same. It is probable that in general 
the basalt flows coincided in direction with the principal val- 
leys of the time. The greatest linear extension of the great 
flows from the "Volcano" cone were southward. At Embudo 
60 miles from the cone, there are evidences at present of several 
distinct flows. At this point the flows were narrow and thin 
and now they cover the high land while both to the east and 
west there are deep valleys in the sand which has never been 
lava-capped. It seems therefore that the present southward 
extension of the lava points to the existence during the basalt 
period, of a river valley flowing southward through Embudo. 
The Rio Grande of to-day flows along this same course. Like 
all superimposed rivers it has been rendered liable by super- 
imposition to certain accidents which present unnatural bar- 
riers for it to overcome. About ten miles above Embudo, for 
instance, the Rio Grande has, in cutting through the lava and 
sands, found in its course a buried hill of quartzite and schist ; 
