VARIATIONS IN GRANITE-PORPHYRY. 227 



structural changes. With this change in structure the rock becomes more 

 compact and weathers in angular blocks with smooth surfaces, the contact 

 products offering the greatest possible contrast with the central portion of 

 the dike, which weathers in rounded masses with rough surfaces, disintegra- 

 ting easily under atmospheric influences. Yet in the wider dikes these 

 changes can be traced so readily, step by step, from the one rock into the 

 other, that the evidence is clear that they are but different structural 

 developments of the same erupted material, but not necessarily identical in 

 chemical composition at the time crystallization took place. In crossing 

 the dikes one passes within 100 yards, over excellent quartz-porphyry, on to 

 normal granite-porphyry, and then on to a rock which can not be told from 

 many varieties of granite ; so that one is forced to believe that the only differ- 

 ences between granite and granite-porphyry is in many cases purely one of 

 structure, dependent upon conditions in cooling rather than upon any 

 differences in age or chemical constitution of the original magma. 



A study of the dikes makes it evident that there could not have been 

 any forcing of lava into the middle of a dike already partially occupied 

 by an earlier crystalline rock. As the branch dikes are mostly narrow, the 

 granitic and normal granite-porphyry structures are less fully developed 

 and are frequently wanting, the effects of chilling and rapid cooling from 

 both walls toward the interior producing only the types of quartz-porphyry 

 developed along the walls of the broader dike. Another striking feature of 

 the rapid cooling of the magma is seen in the marked tendency of the 

 crystalline rock to develop a jointed structure near the lines of contact, in 

 planes parallel to the walls. 



In places the porphyry contacts present a fissile, sherdy structure, 

 lines of parting becoming wider and wider apart toward the center of the 

 dikes and gradually disappearing. In the jointed portion the rock is always 

 fine grained, and frequently possesses an aphanitic structure, the mineral 

 components, however, remaining the same. The rock frequently undergoes 

 marked changes in color in passing from the coarse grained granitic structure 

 to the contact rock. In these changes it will frequently pass from light 

 gray into dark gray, blue, and along the contact becoming almost black. 



