THE EXTRUSIVE PROCESSES. 



611 



tute a large part of the final deposit. These mud-flows chiefly lodge on 

 the lower slopes of the volcano or adjacent to its base, and give rise to 

 rather flat cones, sometimes designated as tufa-cones to distinguish them 

 from cinder-cones formed by the direct fall of fragmental material. 

 Mud-flows appear also to be formed by the ejection of mud and water 

 that had gathered in quiescent craters during intervals between stages 

 of eruption. 



A portion of the finer exploded material floats away in the air to 

 greater or less distances, and forms widespread tufa-deposits. In some 

 cases beds of volcanic ash of appreciable thickness (as those of Nebraska)* 



Fig. 468. — Mt. Shasta, a typical extinct cone, furrowed by erosion, but retaining its 

 general form. (Diller, U. S, Geol. Surv.) 



are found far from any known volcanic center. The extremely fine 

 ash from the great explosion of Krakatoa floated several times around 

 the earth in the equatorial belt and spread northward into the temperate 

 zones. 



1 Ante, p. 22. 



