76 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. - 
the first of which the magma did not reach the surface, being stored in 
a reservoir long enough to develop the large crystals, and later was 
extruded as a viscous paste which did not flow far. There are a number 
of extrusions of this unusual rock within 50 miles of Tucson, indicating 
that this interrupted and renewed movement is a common phenomenon 
in the later igneous extrusions of the district. The structural relations, 
thickness, etc., are best shown in the sections A~B and G-H. ‘The source 
of the flow was probably south of Sentinel Hill, and it is not unlikely 
hidden under the alluvium in the downfaulted portion southeast of the 
indicated fault-line. The lower portion is somewhat scoriaceous, and 
the original upper surface of the lava was blown to small angular frag- 
ments (less than an inch in diameter) by the stream driven outward by the 
crystallization of the semiviscous mass. There are a few inches of this 
breecia preserved on top of the plagioclase-basalt by the next lava flow. 
Following closely, the intrusion of the neck B, and the correlated 
flow B, occurred, if my interpretations are correct. That the interval 
was short is shown by the accumulation of 80 feet of the plagioclase- 
basalt breccia on the top of B,, which was washed down from the surface 
of the plagioclase-basalt now covered by alluvium. Had this interval 
been long, this fragmented material would have been swept off from its 
exposed position on top of the plagioclase-basalt flow. The neck B, 
(the second variety of amygdaloidal basalt microscopically described) 
shows marked variation in both structure and composition. In the 
center and along the northern edge of the exposure it is coarsely crys- 
talline, assuming an andesite structure. In other places it is a very dense, 
heavy rock, entirely cryptocrystalline, considerably altered, the alteration 
products indicating the large iron content of the rock. In places the 
neck is brecciated and the fragments cemented with jasper, and again 
it contains masses of inflated scoriaceous material which decomposes 
into a red or yellow clay. The flow from the above center is the black 
basalt (B,) which covers a considerable portion of the map. It is the 
most basic rock of the sheet, containing olivine visible to the naked eye 
as brilliant red specks formed by alteration. It is classified as an olivine 
basalt. The structural relations are best shown in the section C—D. 
Following this flow there was a short pause in the volcanic activities 
and the neck suffered some erosion, and then the conglomerate was 
deposited (marked “older wash” and shown south of Tumamoc Hill). 
It is 100 feet in maximum thickness and is overlaid conformably by a 
thyolite-tuff 50 to 100 feet thick. At the time of its deposition the con- 
glomerate was the eastern edge of the Tucson slope, and where it was 
thickest it represents the bed of a temporary stream. Later it was faulted 
up to its present position. Its composition and structure reveal its torren- 
tial origin. It consists of beds of andesite and occasional basalt pebbles, 
in a matrix of fine andesite sand. In size the material ranges from small 
