i?. A. Daly — Mechanics of Igneous Intrusion. 295 



Summary. 



A general summary will be of value in gathering up the 

 various threads of the argument tending to support the 

 hypothesis of magmatic overhead st oping. The fundamental 

 facts of the field and laboratory will be briefly recapitulated. 

 There will follow a resume of the tests which have so far been 

 applied to the hypothesis. In those tests, it is believed, may 

 be found a material strengthening of faith in the hypothesis, 

 since it is seen that it explains several principal penological 

 facts not necessarily, or, at least directly, connected with the 

 idea of stoping. 



The fads of the field. — 1. Many, and perhaps most, stocks, 

 so-called " batholiths," and " central granites " show an almost 

 entire lack of sympathy between the structural planes in the 

 invaded formations and the form of the intrusive body. 



2. For some of these bodies there is conclusive evidence that 

 their respective magmatic chambers were not prepared for 

 intrusion by circumferential faulting. For the great majority 

 of the remainder, the indications for such faulting is negative. 



3. For many, perhaps most, stocks and " batholiths," the 

 combined contact phenomena demonstrate some kind of active 

 assimilation of their corresponding country rocks by the respec- 

 tive magmas. 



4. In the normal stock and " batholith," there are usually : a 

 decided lack of any enrichment of the endomorphic zone by sub- 

 stance dissolved from the invaded formations ; a general free- 

 dom from foreign inclusions in the interior, together with a 

 characteristic abundance of angular enclosures near the contacts ; 

 an exceedingly sharp line of contact with the country rocks ; 

 equally sharp contacts of the foreign fragments and their 

 respective hosts ; lack of direct sympathy between the compo- 

 sition of the intrusive bodies and their respective country 

 rocks ; a general high degree of homogeneity in the composi- 

 tion of the igneous body ; the common occurrence of many 

 long and narrow apophyses from the igneous body, indicating 

 strong liquidity at the time of the intrusion of the main igne- 

 ous mass. 



5. Field relations, coupled with a comparison of the chem- 

 ical and mineralogical characters of igneous rocks the world 

 over, show well established and wide reaching laws of mag- 

 matic differentiation. 



6. Isolated observations in nature prove that solid rocks may 

 sink in molten lavas because of differential density ; others 

 show that fragments of solid rock can be more or less com- 

 pletely dissolved in molten lava. 



