THE ORIGIN AND DEVELOPMENT OF THE EARTH 113 
FIGURE 2 
A section of the earth on the basis of the Planetesimal Hypothesis 
(after Chamberlin and Salisbury). The inner division 1, represents 
original planetesimal matter, with some igneous rocks. The second 
zone 2, represents the times of earliest sedimentation. Planetesimal 
matter still dominates, but there is much igneous rock, and some sedi- 
mentary rocks, now changed by pressure and other agencies. 3 is a zone 
rocks, now much changed by pressure and other agencies. 3 is a zone 
of lavas and other igneous rocks, largely volcanic, with some sedimen- 
tary rocks. It represents the deposits of the time when planetesimals 
‘became few and small, but the pressure of the growing earth caused 
much vuleanism. 4 is the newer rocks, mostly sedimentary, represent- 
ing deposits made from the times of the earliest known abundant fossils 
to the present. 
