Senry Woodward — On Volcanos. 339 



This diagram represents 50 miles only out of tlie 3,962 miles from 

 the exterior to the centre, which would require a diagram nearly 

 eighty times as long to express. From this we can easily perceive 

 that unless we watch Nature's great agents at work, and take note 

 of her records, we should know next to nothing of the interior of 

 our globe. We are, in fact, reduced to make the same kind of 

 observations which led Columbus to discover the New World ; we 

 must watch the waifs and strays thrown up by the tide from this 

 great unknown region, 7,925 miles across, which divides us from the 

 Antipodes.. 



The evidences afforded of the condition of the earth's interior are 

 derived from — 1, volcanic products ; 2, thermal springs ; and 3, the 

 observations we are enabled to make of underground temperatures. 



The rocks at the surface of the earth are everywhere roughly 

 divisible into Igneous and Sedimentary, i.e., produced by the agency 

 of fire and water. Igneous deposits, like sedimentary beds, often 

 show stratification ; often, too, they are arranged by currents under 

 the sea ; or they may be arranged in layers subaerially. 



The Sedimentary rocks are all deposited by water, either in lakes, 

 rivers, or in seas. 



All over the world one meets with conical-looking mountains, fre- 

 quently surrounded, more or less completely, by an outer and larger 

 circle of cliffs, and having a crater formed hollow within. (Fig. 2.) 



Fig. 2. — Ideal Section of a Yoleano. 



When we examine their structure, we find they are composed of 

 sloping layers of ashes, cinders, stones, pumice, and other loose ma- 

 terials interstratified with beds of lava and other matter, evidently 

 forced up in a fluid state from within. 



Some of these mountains are still active, and by seeing what goes 

 on at their openings one can more readily understand the origin of 

 other similar hills now long since silent. 



Taking a map of the world we can trace out upon it roughly the 

 chief volcanic regions where active eruptions are going on or have 

 been seen in historic times.^ 



Upwards of 400 volcanic mountains are known to exist in various. 



^ For a world-map of volcanos see Scrope's invaluable work, on volcanos; see alsO' 

 Keith, Johnston's Physical Atlas. 



