170 
IOWA ACADEMY OF SCIENCE Vol. XXIV, 1917 
In concluding* this outline of the progress of thought regard- 
ing geologic history I am reminded of Tennyson’s beautiful and 
expressive lines : 
“There rolls the deep where grew the tree. 
O earth what changes hasythou seen! 
There where the long street roars hath been 
The stillness of the central sea. 
“The hills are shadows, and they flow 
From form to form and nothing stands; 
They melt like mists, the solid lands, 
Like clouds they shape' themselves and go.” 
Iowa Geological Survey, 
Des Moines 
