252 
FRAMEWORK OF ARIZONA 
Only the sedimentary rock formations of the State are thus far 
considered. These give us chief clues to our geological chron¬ 
ology. An equally primiere group of the volcanic rocks are dis¬ 
played in infinite variety, in vast areal extent, and of almost all 
geological ages from the Azoic to the present time. Arizona is 
one of the most profitable fields for petrographic inquiry that the 
world today affords. 
Around the base of the broad Colorado Dome there is one of 
the great lava tracts of Earth; one second in magnitude in our 
country only to the famous northwestern lava plains of Wash¬ 
ington and Idaho, and fifteen times larger than that classical 
Auvergne district of extinct volcanoes in central France. Sweep¬ 
ing in a broad crescent, 250 miles long, with famed San Mateo 
Mount on one horn and the lofty San Francisco Peak on the 
other, the main body of lava flows, superposed in countless num¬ 
bers, covers an area half the size of New York State.^ 
Beyond the borders of the crescent are myriad cinder cones, 
coulees, and limited basalt sheets, which spread out over the less 
resistant sedimentaries, protect the latter from rapid destruction, 
and give relief expression in characteristic plateau plains of the 
desert. Other volcanic evidences are the denuded necks and 
dikes. With this vast Tertic vulcanism must be connected most 
of the metallic ore deposits, as Lindgren more recently also notes. 
Few places are there that would yield larger results on the con¬ 
sanguinity of eruptive rocks. Even the ordinary black traps 
which constitute the mal pais of so many parts of the State are 
not without great interest. As bearing upon these rarer potassic 
magmas a large new preserve is opened up in this country, com¬ 
parable to the best known fields of central Europe.^ These 1am- 
prophyric basalts of Arizona doubtless represent the last stages 
of eruption of alkalic magmas which are yet uncovered by erosion. 
Pirsson’s comment that “It is clear that here is a field whose 
% 
study, especially on the chemical side, will yield rich results to 
detailed petrographic investigation,’’ is strongly endorsed. 
1 Bull. Geol. Soc. America, Vol. XXIII, p. 715, 1912. 
2 See: S. Richarz, this journal, Vol. XXXVIII, p. 112, 1922. 
