CONCLUSIONS 331 



QtTAETZ-BEAEING IGNEOUS EOCKS 



As already noted, the quartz-bearing lavas, Class 8 of Table III, are 

 probably confined to the region of continental fragmentation in the south- 

 west Pacific. Their origin is a world problem specially affecting the 

 geology of the continents, and the Pacific islands are not likely to shed 

 much light on that complex subject. Yet in island or mainland one of 

 the chief facts to be considered is the general association of feldspar 

 basalt or its chemical equivalent with quartz-bearing lavas, where the 

 latter types do occur. That fact is explained by the syntectic-differentia- 

 tion theory of igneous rocks. No other explanation covering the ascer- 

 tained field, chemical, and mineralogical relations has yet been published. 



Miscellaneous Types 



The species listed in Class 9 have uncertain affinities. Most of them 

 are chemically related to basalt or ordinary gabbro, and their genesis 

 probably involves no important principle not represented in the origin of 

 Classes 2 to 7, inclusive. 



Conclusions 



The available petrographic data do not yet, of course, permit of final 

 conclusions as to rock origins, but they do suggest : ( 1 ) that underneath 

 the Pacific the only primary magma is, and long has been, of basaltic 

 composition; (2) tha.t the pyroxene andesites and picritic types are direct 

 differentiates of this primary magma itself; (3) that the alkaline rocks 

 may possibly be due to the solution of comparatively small proportions 

 of limestone in the primary basalt, the syntexis being usually masked by 

 differentiation. 



The primary basalt may most simply be conceived as forming a con- 

 tinuous stratum beneath the whole ocean basin. From this subcrustal 

 stratum material has been eruptible, locally and from time to time. The 

 failure of quartz-bearing lavas in most of the basin suggests that the 

 basaltic substratum is there not overlain by a solid quartzose crust, as it 

 is in continental areas. With this conception may be correlated the 

 geodetic proofs of specially high density beneath the Pacific Ocean. 



Suggestion and hypothesis have great value if they lead to action, to 

 renewed search in nature for the facts. A glance at the larger aspects 

 of Pacific petrology shows how pitifully slight is our knowledge of the 

 island petrography. Now is not the time for settled convictions. Now 

 is the time for concerted, persistent effort, leading to a thorough explora- 

 tion of the Pacific archipelagos, under the auspices of a single institution 

 with a. staff of cooperating observers. 



