78 PHILOSOPHICAL SOCIETY OF WASHINGTON. 
way their activity is inferred to have prevailed in a period not far 
from middle tertiary time—possibly in the mioeene. The large 
amount of erosion which has occurred since their eruptions ceased 
forbids a much later period, and the still larger amount of tertiary 
erosion which preceded this activity equally forbids a much earlier 
one. . 
Upon the summits of the mesas no recent eruptive rocks occur. 
But in the broad valleys which lie between them and around them 
are lavas of quite another age. These valley lavas are all recent. 
Indeed the most superficial observer is at once impressed with the 
freshness of their aspect, and critical examination confirms the 
view that none of them have any geologic antiquity, while some of 
them are so modern that it seems as if half a dozen centuries were 
a large estimate of the time which separates us from their outflow. 
These recent eruptions are basalts of normal type. The external 
aspects of the fields of young lava resemble those of the Hawaiian 
Islands. The two forms of solidified lava are well presented, viz: 
the viscous or ropy, and the rough clinker fields. 
A striking characteristic of both old and young lavas—those 
upon the mesa summits and those in the valleys below—is the usual 
though not universal absence of cinder cones or piles of fragmental 
matter built up around the orifices from which the lavas were ex- 
truded. The eruptions, with the exception of those of Mt. Taylor, 
belonged to the quiet order which are typified among volcanoes 
now active, by Mauna Loa and Kilauea. 
But the volcanic remnants which appeal most strongly to the im- 
agination of the observer, remain to be described. In the broad 
valleys which separate the lava-capped mesas are seen many con- 
spicuous objects rising as sharp peaks or aiguilles of rock to great 
altitudes. They are very black in color, and contrast powerfully 
with the bright tints of the sedimentary beds around them. These 
peaks, which range in altitude above the valley plains from 
700 or 800 feet to 2,000 feet, consist of columnar basalt. They 
are, in fact, the ancient lavas which congealed in the volcanic 
pipes, while the sedimentary strata which formerly inclosed them 
have been swept away in the great erosion of the country. In that 
long-continued and great denudation these “necks,” by their more 
adamantine character, have resisted the general decayyand remain 
to attest the former extension of the strata over the valleys and the 
