86o PRINCIPLES OF STRATIGRAPHY 



the latter case direct observation of such manifestations is im- 

 possible, but their characters may be inferred from the results of 

 past intrusive and plutonic manifestations as indicated by the char- 

 acteristics of intrusive and deep-seated (plutonic) igneous masses. 

 (See ante. Chapter \'II.) 



Extrusive manifestations may further be divided into the ter- 

 restrial and the submarine, the latter again being vv^thdrawn from 

 direct observation, except when their results appear above the 

 surface of the sea, after prolonged existence. Indirect observation 

 on the results of submarine eruptions is likewise scanty, mainly 

 because, in the case of older volcanics, it is at present difficult to 

 distinguish with certainty between submarine and subaerial erup- 

 tions, and many so-called stibmarine lava flows must probably be 

 relegated to the subaerial type. 



In discussing the types of eruptions, the primary division into 

 explosive and extravasative will be kept in mind, and under each 

 of these will be noted the subaerial or terrestrial and the submarine 

 types. 



Explosive Eruptions. 



Terrestrial Type. Volcanoes of purely explosive type are prob- 

 ably very rare, though the "iliaare" craters may be classed here. In 

 the typical examples of the Eifeler Maare, no volcanic cone exists; 

 instead, there is merely a more or less circular opening, the result 

 of the explosion, and this has subsequently been filled with water. 

 Lapilli, bomblets, and even large bombs often abound in the neigh- 

 borhood of the -maare craters, but lava flows are typically absent. 

 The coarse and fine lapilli of the maare region in the Eifel and 

 Rhein districts form stratified deposits which have all the appear- 

 ance of stratified sands of clastic origin. As outlined in Chapter 

 VI, the lapilli are not to be regarded as pyroclastic in the true sense 

 of the word, but rather as granular pyrogenics, being primarily of 

 endogenetic origin, and classifiable as pyrogranulites rather than 

 as pyrarenytes. True pyroclastics are, of course, also associated 

 with the deposits of lapilli and bombs, these resulting from 

 the clastation, by eruptive explosion, of already consolidated 

 rock masses, either of igneous or of "sedimentary" origin. The 

 vicinity of the Laacher See, the largest and most picturesque of 

 the explosive craters of Germany, is characterized by deposits of 

 volcanic bombs and tufifs, of trachyte, mingled with clastic frag- 

 ments of granite and various metamorphic rocks, brought up by 

 the explosion from great depths below the cover of Devonic strata. 



