Geology of Albuquerque, N. M. — Herrick. 35 
The north and south length of the area is about three or four 
miles and the greatest thickness of the lava is not more than 
twenty-five feet at any point seen, though the thickness is not 
at all constant, but in some places is reduced to four or five 
feet. The appearance of the upper surface shows that there 
was not a single wide-sweeping flow, but a somewhat inter- 
mittent series following in such a way that the older flows 
were in some cases overtopped by later discharges of the same 
material. In composition the lava is apparently exactly like 
that of the entire series of recent basalts of New Mexico, 
which will form the subject of a separate paper. A few miles 
further west is a second larger cone, the flows from which 
may prove confluent with that of the one now under consid- 
eration on the west. It is noticeable that these recent cones 
tend to be clustered in groups. The present group may be 
known as the Isleta group, comprising an east and west Is- 
leta peak. It lies about ten miles sotith of the Albuquerque 
group. But the interest which attaches to this flow is quite 
apart from the character of the lava itself. As already indi- 
cated, it would not be unnatural to infer, in looking at the 
deposits as they lie in the valley apparently parallel to those 
formed by the river to suppose that these two are essentially 
the same — fl.viviatile sand and gravel accumulated by the river 
at a time when it was larger than at present, or when its bed 
was less inclined toward the south. If this were the case it 
would follow that the lava flows might be comparatively re- 
cent and such vestiges of man as might be found beneath the 
lava need not be assigned any great antiquity. But a brief 
study of the sub-igneous deposits in the Albuquerque group 
of cones seemed to point to a quite different conclusion. If 
they were really fluviatile these deposits were not a part of 
what has been called the Albuquerque series, for they are of a 
different lithological character. In the modern river gravels 
the pebbles are of gneiss and granite as well as quartzyte, 
but in addition contain great numbers of fragments of an- 
desyte, rhyolyte and trachyte, i. e., of materials such as are 
found in the older eruptives of the valley. So far, these last- 
named elements have not been found in the sub-igneous de- 
posits mentioned, but fragments such as may have come from 
the Cretaceous of the region to the west, if not of that age 
