36 The American Geologist. July, i898 
themselves, are commingled with the granitic debris. But in 
the midst of the perplexity growing out of the evidence that 
the river erosion of the time when the present gravels were 
deposited has involved these lava-topped bluffs, the study 
of the material below the lava of the Isleta cones has offered 
an unambiguous solution of part of the problem. At that 
place indeed the recent river gravels (No. 2 of the Albuquer- 
que series) are in places to be seen actually overlying the lava 
and forming extensive beds upon its surface. On the other 
hand, as stated elsewhere, fragments of the lava and of pumice 
have been found in No. 2 on the opposite side of the river, 
An examination of the materials below the flow at Isleta 
shows that they are largely composed of sand grains of ap- 
parently granitic origin. They are mostly quite fine and only 
rarely do large pebbles occur. Close inspection shows that 
in many places there is a large admixture of small black grains 
of a rather obscure nature. Fortunately a place was found at 
a point nearly due east of the crater where the whole mat- 
ter is explained. Here the thickness of the underlying grav- 
els is exposed for about fifty feet, and the lava is very irregu- 
lar. In some places it is fifteen to twenty feet thick and in an 
adjacent part it is less than five, while in other places the lava 
never has flowed over the gravel beds. The latter, in those 
portions in which they are, or have been, covered by the lava, 
conforms to the irregularities of the under-surface of the flow 
so that, as the deposits are very clearly and minutely stratified, 
there is no question of erosion prior to the flow suf^cient to 
account for the irregularities. Different parts of the stratified 
gravels are unconformable to each other, as might be in a 
current of changeable character with great burden of detritus. 
But the clue to the nature of these deposits is found in the fact 
that in the midst of this stratified deposit and scattered 
through it from top to bottom of the entire exposure are large 
angular blocks of basalt quite like the material of the flow and 
varying in size from the diameter of one's head to that of a 
pea, arranged irrespective of stratification, though in some 
places there are bands filled with small grains of a similar 
character. The appearance of the exposures is curious in 
the extreme for the mass is so minutely stratified and its ma- 
terials so plainly the detritus of sedimentary deposits that the 
