102 The American Geologist, September, iso? 
best aid to a conception of geological time is geological field 
work. One who sees erosion going on in the park at h slow 
rate and measures the work already completed has a time 
measure. When he notices the different great systems of sed- 
imentary rock and the many varieties of eruptive rocks, he 
obtains a series of objects that measure back in sharply de- 
fined units to the remote millions — fading awa}^ in the distance 
like a series of telegraph poles. 
Thus to properly understand the causes of the present ap- 
pearance of the park it must be kept in mind that there have 
been definite periods in its making. After the first long peri- 
od was completed by profound disturbance and a series of 
eruptions and long continued erosion in the Eocene, there 
came in Miocene time three eruptions of well defined character. 
The first was an eruption of moderately acid rock called "an- 
desite." It is a light colored rock of various shades. Its 
principal bulk or groundmass is glassy to cryptocr3^stalline 
and contains phenocrysts of three minerals, viz.. a soda lime 
feldspar (andesine or labradorite), hornblende or augite, and 
biotite. This rock forms the tops of many of the mountains 
and is called the scenery rock. After a long period of cool- 
ing, erosion and cracking, came an eruption of an acid rock, 
rhyolite. It also is a rock of varied colors and texture, vary- 
ing from white to black and from glassy to cryptocrystalline. 
Its phenocrysts however are chiefly a soda-potash feldspar and 
quartz. It covered the whole park, filling all the valleys and 
rising two thousand feet upon the sides of the mountains. 
About the only rock that the tourist meets in the park is rhy- 
olite. 
The eruption of rhyolite was followed b}^ one of a dark bas- 
ic rock, or basalt. It is a dark-colored glassy or completely 
crystalline rock with groundmass of labradorite and pheno- 
crysts of lime feldspar (anorthite) and augite and oli- 
vine. 
The order in which these rocks were erupted is instructive. 
Prof. Iddings calls attention to the fact that first there came 
forth a rock of mixed chemical composition. After the molten 
mass or magma in the earth had time to differentiate, the ba- 
sic or heavy material sank and the acidic rhyolite arose near 
to the surface. The next faulting forced this top layer out 
