COMMON SOURCE OF LAVAS. 267 



olivine. In general both alkalies :nay be said to decrease from the acidic 

 toward the basic end, and, except in the more basic basalt, the potash 

 exceeds the soda in amount. 



There is a much gi eater range throughout the entire series of lavas in 

 the percentage of potash than in that of soda, the former showing a varia- 

 tion of over 3'25 and the latter of only 1/50 per cent. The greatest inter- 

 ruption in the regularity of the potash is shown along the line where the 

 sanidine disappears and some one or more of the lime-soda feldspars become 

 the predominant species, whereas with the soda no such break is noticeable. 

 In the liquid mass, under influences very little understood, the material 

 forming ferro-magnesian minerals draws apart from the alkalies and excess 

 of soda, the result of which is to produce separate magmas differing widely 

 in chemical composition. 



Common Source of Lavas. In the preceding pages all the extravasated lavas 

 have been considered as belonging to one or the other of two distinct 

 magmas, yet it is impossible, notwithstanding they are so sharply con- 

 trasted in certain fundamental structural characters, not to recognize the 

 fact that both magmas stand in the closest relationship to each other. The 

 similarity in mineral development as they approach each other in chemical 

 constitution, the gradual changes in the relative proportions of the oxides of 

 the different elements throughout the entire range of lavas, show how close 

 a connection exists between them. An equally strong argument is found 

 in their geological distribution, where the rhyolite occurs closing up the 

 vents occupied by the feldspathic magma and at the same time breaking 

 out as the earliest eruptions along fissures which later served as channels 

 for the pyroxenic magma. The loci of eruption of both magmas have been 

 shown to be in close proximity to each other, and some of the most acid 

 and most basic lavas, so far as external evidence can determine, not only 

 reached the surface along the same great fractures, but actually used the 

 same conduits at a number of localities. 



To the writer, after studying all the facts, it seems impossible to regard 

 these differentiated volcanic products otherwise than as belonging orig- 

 inally to one and the same body of molten material; in other words, they 

 were derived from a common reservoir. To conceive of such a separation 



