LACCOLITHIC STRUCTURES 
119 
likelihood the Dolores reef is identical, being an ancient already 
metamorphosed plate instead of a more modern Cretacic sandstone. 
This being the case it is quite possible that at Dolores it is a part 
of the floor of the Ortiz laccolith that is actually exposed. 
Complexity of Los Cerrillos Hills. In the other groups of the 
Sierra del Oro only single masses constitute the laccoliths. Los 
Cerrillos group differs in having besides a main intrusive body 
numerous subordinate lenses that occupy spaces between parted 
strata, particularly in situations where the beds are sharply flexed. 
A particularly noticeable feature of the associated fault-plane is 
that in the same vertical line the displacement below the laccolithic 
body is only about 1000 feet while above the intrusive mass it is 
nearly 3,000 feet. A cross-section is represented annexed (figure 
10). 
Depth of Formation. The Sierra del Oro presents rather unique 
aspect for reason of the fact that it directs attention to the depths 
at which laccolithic intrusion takes place. This phase of the 
problem receives mention no where in connection with descrip¬ 
tions of this class of mountains. The discussion hinges on the 
nature of contact copper deposits; and the different instrusions 
seem to furnish the necessary evidence. 
From experimental results on the conditions under which 
garnet-rock (the contact copper matrix) forms Chrustschoff 
reached the conclusion that the temperatures reaching 550 C. were 
indicated. High pressures were premised but no definite figures 
ventured. On this point Van Hise, reasoning from these physical 
conditions, argues for the formation of true contact ore deposits 
as possible in the zone of rock-flowage. This was placed at 
depths of from four to eight miles, where the temperatures were 
above that of the critical point of water, and the pressure was 
above 200 atmospheres. 
In the Sierra del Oro we seem to have some exact figures for 
the thickness of the rock-pile above the contact ores, and hence 
for the thickness of the laccolithic cover. The laccolithic intrus¬ 
ions have entered at stratigraphic horizons which are definitely 
located in the local geologic column. The regional rock-succes¬ 
sion is that of the west side of the Rocky Mountains rather than 
that of the better known east-side section. The general sequence 
of the rock formations and the maximum thicknesses represented 
in this area are as follows: 
