academy of sconces] DISCUSSION AND CONCLUSIONS. 75 



Bowen's suggestions mentioned above. The presence of so huge a mass of peridotite injected 

 as such into Tertiary rocks seems to show the possibility that such intrusions have been much 

 more frequent than recent discussion of peridotite might suggest, and thus supports Dr. Harker's 

 hypothesis of successive intrusions of decreasing basicity. 



A WORKING HYPOTHESIS OF TECTONIC PETROLOGY. 



This leads us to further speculation. It is manifest that no general petrogenic hypothesis 

 can be well founded that is not supported by a study of the tectonic and other conditions ac- 

 companying the extrusion and intrusion of all types of igneous rocks, at least as complete as is 

 here attempted for the ultrabasic and basic intrusive rocks only. The writer has not made so 

 extensive a study, yet since the further discussion of the problem of the tectonic conditions 

 accompanying the intrusion of basic and ultrabasic rocks can not be advanced without a general 

 petrogenic application, we may here outline tentatively a working hypothesis for the purpose 

 of grouping available facts into some significance. It will be obvious that most of the concep- 

 tions therein are due to others. 



A very general conception is that of batholiths as essentially stratiform masses of magma 

 rising from a potential magma-zone at great depth in a more or less oblique course, and aided in 

 some degree by " overhead stoping " (Harker '09, Iddings'14). From these " cupolas " may arise 

 and extend laterally into great laccolitic masses. While the magma in the stratiform reservoir or 

 in the laccolite, if present, remains undisturbed, gravitational differentiation may perhaps produce 

 a series of associated rocks, stratified according to density. We must recognize that such occur 

 among plutonic masses now exposed, though Harker ('16) and Cross ('15) concur in believing 

 indubitable examples to be comparatively rare and generally indicative of special conditions 

 of fluidity. Where small movements have occurred during consolidation, the appearance of 

 successive intrusion may be produced, as argued by Daly and Bowen. Where greater displace- 

 ments of the differentiates occur, the conditions pass into those of the drafting-off of successive 

 fractions from the primary and perhaps secondary derived reservoirs. Whether such successive 

 intrusions are closely associated or not — that is, whether they rose along similar or diverse 

 paths — depended on the varying tectonic conditions, and the extent to which crystallization 

 had blocked the channel followed by the preceding magma. The earlier basic and especially 

 ultrabasic intrusions in any epoch of vulcanicity accompanying orogeny were generally injected 

 into the planes of structural weakness, and form stratiform intrusions, often with marked flow- 

 structure, perhaps caused by their being in very large measure crystalline at the time of their 

 injection. Some gravitational differentiation may occur in these, with, perhaps, further develop- 

 ment of banded structure in the manner described by Bowen ('19), and the production of an 

 upper layer in which liquid predominated, and a lower of more or less closely knit crystals. If, 

 in the absence of marked lateral pressure, further intrusion occurred shortly after the first, 

 the path of least resistance would naturally be found in the upper layer, and the new magma 

 injected into this might mingle more or less intimately with the upper layer of the older intru- 

 sion. If, as so frequently appears to be the case, the successive drafts of magma ejected from 

 the subcrustal reservoir were increasingly acid, the appearance of gravitational differentiation 

 would thus be produced as the laccolite swelled, being filled out with a series of intrusions. 

 But if a pause occurred between successive intrusions sufficient to permit the formation of a 

 strong crystal-mesh or complete crystallization in the upper layer and the feeding-channel, the 

 latter magma may be injected below the "solidified cake" of the former intrusion, may envelope 

 it, or may break through it in batholithic fashion, perhaps with the local production of hybrid 

 types. On the other hand, if the injection of the magma resulted from " a single principal act" 

 and was so rapid that no noteworthy differentiation occurred during its progress, the differen- 

 tiation of the originally almost homogeneous magma, and the apparent passage or injection 

 of the several differentiates into one another, might follow on the lines laid down by Bowen. 

 In places, more than one basic-to-acid series of plutonic rocks are thus developed sometimes 

 with associated effusive rocks and minor intrusions. Where lateral pressure is pronounced, 

 81795°— 26 6 



