EXAMPLES OF GEOLOGICAL REASONING. 147 
3. Igneous Agency.—lIt is evident that volcanoes have a great power in 
throwing out masses of different materials to great distances, and of 
carrying many substances on their mighty lava-streams ; and evidences 
of their power in this respect in geologic times are everywhere apparent. 
Sometimes the fine ashes that issue from the crater are borne by the 
wind to great distances, often fifty or a hundred miles, where they are 
deposited as a layer of finest dust ; and this may account for the existence 
of trap-tuffs in places where no volcanic eruption seems to have taken 
place. 
It thus appears that the agencies now at work on our own globe are 
adequately sufficient to account for all the phenomena of the forming, 
disturbing, disintegrating, elevating, depressing, and transporting of the 
rock-masses that form the crust of the earth. Such being abundantly 
proved, and the laws of nature being uniform and unchangeable, we 
are not only warranted, but compelled, to infer that the same influences 
were at work in these bygone ages, and were the joint causes of the 
formation of our rock-systems as they are now presented to our eyes and 
subjected to our investigation. 
Examples of Geological Reasoning. 
: 
We shall now give some examples of the method of reasoning on the 
various phenomena presented in the rocks, the formation and history of 
which it is the province of Geology to unfold. 
Reasoning regarding Rock Sections. 
1. Section with Igneous Action and Dislocation —From the section (fig. 81) 
let us see what we can discover, by the mutual relations of the rocks, 
re 
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regarding the times of their formation and dislocation. Which rock was 
first deposited in this section? The lower rocks were of course formed 
