Geological Rcconnoissance in N. M. — Hernck. 333 
acteristic effect upon the clays of the region, as the native pot- 
ters have apparently learned to their sorrow. To illustrate 
this point we may digress a moment. Specimens of Indian 
corn imbedded in what has generally been called lava have been 
found by various persons in the vicinity of the mountains and 
the theory has gained credence, even in professional circles, 
that there were cases whe'"e the pueblo buildings had been 
covered by the lava and tliat the corn found came from pre- 
historic graneries. Specimens brought to us were of so de- 
ceptive a nature that several expert geologists pronounced 
the material lava without hesitation. The writer, however, has 
been so thoroughly convinced that all the acid lavas of the 
region here covered are older than the basalts that he hesi- 
tated till the locality could be visited. Meanwhile the analy- 
sis of the supposed lava showed it to be acid with a composi- 
tion somewhat like obsidian but not typical. The composi- 
tion was more like a slag from a grate. It was found after 
long search that the corn occurred in a ruined pueblo distant 
from any lava flow and that the material was simply vitrified 
adobe. The potter in attempting to use the highly alkaline 
adobe found that a small heat fused the material. In another 
instance on visiting a reputed prehistoric snielter we found 
the brick completly vitrified and on incjuiry learned that the 
ruin was a part of a brick kiln erected a few years ago and 
abandoned because the clay vitrified and could not be used for 
the purposes intended. The information one receives re- 
specting archaeological matters in the south-west it is well to 
accept cum grano salis. 
Along the western bank of the Rio Grande for the whole 
extent shown in the map, is an extensive exposure of Neocene 
Tertiary sand, the so-called Santa Fe marl. As a matter of 
fact the marl is a thin sheet not exceeding twenty feet thick, 
reposing on several hundred feet of loose sands and gravels. 
It has been called Miocene by Cope, but the evidence seems 
to point to a Pliocene age for at least the upper part of it. 
North of Bernalillo it is covered by an extensive lava sheet. 
The lava is of recent basalt, which may be seen in several 
places penetrating the sands of the Tertiary, as described in 
the American Geologist, July. 1898. 
The Tertiary presents to the river high bluffs as the stream 
