118 
LACCOLITHIC STRUCTURES 
strated than by the structures displayed in the-Ortiz Mountains. 
There, also, it is completely demonstrated that a laccolith is not 
a thickened sheet and that the two are formed under entirely 
distinct physical conditions. 
On the south slope of the Ortiz group the limestone cover of 
the intrusive body is still retained, now turned in places into 
garnet-rock and displaying typical contact deposits of copper. 
The Old Lucas mine finely shows all the phenomena peculiar to 
normal contact ore-bodies.^® 
At the eastern base of the Ortiz Mountains is the site of the old 
Spanish placer camp of Dolores. Running through the camp 
is a wide quartz ridge. The main interest in this ridge is that 
the Dolores miners believed that in it they had discovered the 
great “Mother Lode” —the source of all the gold of the district. 
On this quartz ledge a vast amount of work has been done in days 
gone by. When recently examined it was found to be a white 
quartzite instead of massive quartz, and its origin was casually 
ascribed to a section of one of the thick Cretacic sandstones which 
had been caught in the molten magma and become thoroughly 
metamorphosed. Later, however, an indistinguishable quartzite 
was noted several miles to the westward, in the Tijeras canyon, 
where it also had been thought to be a quartz reef, and had been 
extensively although vainly explored for gold. The late Prof. 
C. L. Herrick thus interpreted its presence. Microscopical 
examination of thin slices of this rock clearly indicates the clastic 
origin.^^ This is the pre-Cambrian Tijeras quartzite. In all 
10 Journal of Geology, Vol. XVI, p. 442, 1908. 
11 Bull. Univ. New Mexico, Vol. I, p. 101, 1899. 
12 Kept. Gov. New Mexico to Sec. of Interior, for 1903, p. 101, 1904. 
