464 F. II Lahee — Metamorphism and Geological Structure. 



the granite, less so near the pegmatites, and least near the 

 quartz veins. 



(8). While the quartz veins are not dynamically metamor- 

 phosed, and the pegmatites have suffered at most only slight 

 crushing, the granite, which represents the oldest phase of the 

 series, locally has prominent gneissic structure. 



— Relations of the intrusion of the acid intrusives to the 

 folding, metamorphism^ and schistosity of the Carboniferous 

 sediments. — We have said (p. 456) that, while the granite may 

 grow finer adjacent to the country rock, it never becomes aph- 

 anitic. Indeed, in some places it shows no diminution of 

 texture. There is no marked sign of chilling. Instead, the 

 phenomena indicate that the country rock was relatively warm 

 at the time of intrusion. 



The marginal zones of the pegmatite dikes are frequently of 

 finer grain than the central regions. This might be explained 

 partly or largely by the influence of the inwardly assembling 

 mineralizers, and, to a less degree, by that of surrounding tem- 

 perature conditions. .Nevertheless, here again, nothing sug- 

 gests a cool country rock. 



Because of the massive nature of the quartz veins, it is 

 impossible to ascertain their relations to the sediments in this 

 particular. 



We conclude that the temperature of the country rock, at 

 the time of intrusion, was high. 



Exomorphic alteration, due to injection of the Acid Intru- 

 sive Series, includes solution, introduction of minerals peculiar 

 to the intrusive, and recrystallization of original constituents 

 of the country rock. These changes are such as to imply the 

 cooperation of considerable heat and pressure, of solvent 

 power, of the action of mineralizing agents, and of long time. 

 They are characteristic and are distinct from the results of 

 dynamic metamorphism, as described in Part II for the entire 

 area investigated, in spite of the fact that this metamorphism 

 is more severe westward and southward. The zone of contact 

 metamorphism is limited to but a few feet in breadth. We 

 must infer that the Acid Intrusive Series, although responsible 

 for some contact metamorphism, did not cause the widespread 

 metamorphism of the basin. 



We noticed that the Acid Intrusive Series exhibits a depend- 

 ence upon certain directional structures (stratification and 

 schistosity) in the country rock, and that this dependence 

 increases southward. It follows from the distribution of the 

 series that the degree of dependence of the different members 

 varies inversely as their degree of acidity. And it follows, 

 further, that this degree of dependence is less in the younger 

 intrusives : for the more acid members, representing an 

 advanced stage of differentiation, are somewhat later in origin. 



