COMMON SOURCE OF LAVAS. 267 



olivine. Ill g'eiicral both alkalies may ]»c said to (Iccrcasc t'roin tlic acidic 

 toward the basic end, and, except in the more basic basalt, the ])otash 

 exceeds the soda in anj<innt. 



There is a much f^i eater range thron<^'hont tluf entire series of la\as in 

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

 tion of over 3'2F) and the latti-r of only 1-50 ])er cent. The greatest inter- 

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

 sanidine disa[)pearsan(l some one or nior(M)f the lime-soda feldspars become 

 the ])redominant species, whereas with the soda no such })reak is noticeable. 

 In tile li(iuid mass, under influences very little understood, the material 

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

 of soda, the result of which is to produce sejiarate magmas difl^ering widely 

 in ciiemical 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 sliarplv con- 

 trasted in certain fundamental structural characters, not to recognize the 

 fact that l)oth magmas stand in the closest relationship to each other. The 

 similarity in mineral development as they ajiproach each other in chemical 

 coiistitiition, tlie gradual changes in the relative proportions of the oxides of 

 the dirt'ereiit elements throughout the entire range of lavas, show how close 

 a connection exists l)et\veen them. An equallv strong argument is found 

 in their geological ilistribution, where the rliyolite occurs closing up the 

 vents occupie<l l)y the feldspathic magma and at the same time breaking 

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

 for tlie j)yroxenic 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 actualh' used the 

 same conduits at a number of localities. 



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

 these differentiated volcanic products t)therw'se than as l)elonging orig- 

 inally to one and the same bod}' of molten matenal; in other words, they 

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



