VARIATIONS IX GRANITE-PORPIIYEY. 227 



structural changes. Witli this chiiuge in structure the rock becomes more 

 compact aud weathers iu anguhir blocks with smootli surfaces, tlie coutact 

 products offering- tlie i>-reatest possible contrast with the central })ortion of 

 the dike, which weathers iu rounded masses with rough surfiices, disintegra- 

 tuig easily under atiuosplicric influences. Yet in the wiiU-r <likes 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 iu 

 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-porphjTy, aud 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 ami granite-porphyry is in many cases purely one of 

 structure, dei)endent upon conditions in cooling rather tlian uptm an}- 

 differences iu 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 midiUe of a dike already partially occupied 

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

 granitic aud normal granite-porphyry structures are less full}' developed 

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

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

 developed along the walls of the l)roader tUke. Another striking feature of 

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

 crvstalline 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, sherd}' structure, 

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

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

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

 components, however, remaining the same. The rock fre([uentl}' undergoes 

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

 to the contact rock. In tlicse (-hanges it will frequently pass from light 

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



