D. THE PYROSPHERE. 

 CHAPTER XXII. 



GENERAL SUMMARY OF PYROSPHERIC ACTIVITIES. 



The activities of the pyrosphere are judged by their surface 

 manifestations, and by the observations on results of igneous ac- 

 tivities in the past. So far as the pyrosphere itself is concerned 

 direct observation is, of course, out of the question, nevertheless 

 much may be learned regarding its probable character by experi- 

 mentation and the study of igneous activities in the laboratory, as 

 well as in the field, while much more may be inferred from a logical 

 interpretation of past igneous work in portions of the earth's crust 

 exposed as a result of dislocations, or of prolonged erosion, or of 

 both. No attempt is made to discuss volcanic activities in anything 

 more than a summary manner, though the subject is of vast geologi- 

 cal importance. The science of pyrology or vulcanology has already 

 developed a literature which only a specialist may hope to master. 

 The list given at the end of this chapter is an extremely fragmentary 

 one, but it contains a sufficient number of general works in which 

 the subject is treated from a comprehensive viewpoint, and which 

 will open for the student the gateways to the special fields of re- 

 search in which ground has been broken. 



VOLCANIC ACTIVITIES. 



TYPES OF VOLCANIC ACTIVITIES. These may be purely 

 explosive or purely extravasative or, what is more frequent, a 

 combination of both in varying proportions. According as the one 

 or the other prevails, the form of the resulting deposit will vary 

 from simply conical in the first to flat and plain-like in the second 

 case. 



Subdivision with Reference to Location. Volcanic mani- 

 festations may take place either oil the surface of the lithosphere 

 (effusive) or within the earth's crust (plutonic, intrusive). In 



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