ORIGIN OP ERUPTIVE AND PRIMARY ROCKS. 353 



suppose further that a change comparatively slight takes place in 

 the position of the metallic centre. It is evident that the conse- 

 quence of this would be to press the higher zone against the 

 solidified crust, and further to each side, bringing the lower zone 

 in contact with the crust at a, as shown in figure 2. If at this 



Fig. 2. 

 part of the crust there existed fissures or volcanoes, the denser 

 and more basic mass, b would be erupted, while the acid 

 mass, c would remain in the interior. As soon, how- 

 ever, as the central mass resumed its normal position the condi- 

 tions would be re-established for the protrusion of the more acid 

 rock of the superior zone. In this way the alternate eruptions of 

 highly silicified matter and then of extremely basic rock with all 

 the innumerable gradations that exist between them, would seem 

 to be capable of explanation. The amount of divergence of the 

 metallic mass from the centre necessary to produce the effect 



