11'^ 



THE POPULAR SCIENCE MONTHLY. 



direction slanting away from the axis, until the beds reach the maxi- 

 mum angle of repose of the rock-fragments in question as the beds 

 (D D) on diagram. Thus we have constructed a conical mass in the 

 center of which is the volcanic chimney (B), and, dipping away on all 

 sides at angles, varying generally between 20 and 45, we find the 

 strata composing the cone (D, E). This arrangement is often called 



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periclinal. The funnel, or chimney, which has been mentioned as 

 occupying the center, has the form of an inverted cone, the inclination 

 of its sides and its diameter necessarily being proportional to the vol- 

 ume and force of the escape of vapor, and also to the nature, form, 

 and size of the surrounding fragments, forming the growing cone, 

 which have already been ejected. The upper, or basin part, is techni- 

 cally called the crater. The vapor only may have made its appear- 



