THE ORIGIN OF THE INCLUSIONS IN DIKES 169 



Inclusions in the dike-rocks are extremely numerous and vary in size from 

 tiny fragments up to masses nearly 3 feet in length. They consist of rounded, 

 subangular, or occasionally angular, fragments of shale, sandstone, granite, 

 metamorphosed limestone, and quartzite conglomerate. The inclusions of 

 sedimentary rock are relatively abundant in approximately the same propor- 

 tions as those in which the shales, sandstones, limestones, and conglomerates 

 occur in the underlying sedimentaries^the Mancos shale, the Dakota sand- 

 stone with its basal conglomerate, the McElmo and LaPlata formations, etc. 

 In the thicker dikes, shale inclusions are more numerous near the contact walls 

 than toward the middle of the dike. The inclusions of sedimentary rocks are 

 evidently fragments from the walls of the conduits through which the lava 

 passed upward and from which they were torn by some process analogous to 

 stoping. They all show the effects of their immersion in the molten mass with 

 which they were surrounded. 



The origin of the granitic inclusions is less apparent. No granites are 

 known to outcrop within the drainage basin of the Mancos River, which 

 includes much of the LaPlata Mountains northeast from Mancos. It is 

 believed, however, that their origin must be similar to that of the other inclu- 

 sions in the same dikes, and hence it is inferred that beneath the sedimentary 

 rocks which outcrop in this region there must be a body of granite which was 

 likewise cut by the dikes and from which fragments were torn by the ascending 

 lavas in the same manner as those from the overlying strata. Such a granite 

 mass might be either an intrusion into the sedimentaries, preceding the vul- 

 canism, the effects of which are now displayed at the surface, or it might be a 

 hill of pre-Cambrian or Paleozoic rock around and above which the Mesozoic 

 sediments were laid down. These inclusions of igneous rock display a remark- 

 able selective assimilation by the dike-magmas of certain of the minerals of the 

 granite. This will be discussed in a paper now in preparation. 



Aschaffenburg, Germany: In the Spessart region near Aschaf- 

 fenburg there are a number of lamprophyre dikes, some of which 

 contain inclusions of granite or of minerals derived from an augen 

 gneiss. In the Schweinheimer kersantite dike, Thiirach^ has 

 found inclusions 40 cm. in diameter, of a granite which contains 

 orthoclase augen 5 cm. in length. Goller^ has described quartz 

 and orthoclase inclusions from similar dikes. The quartz fragments 

 are rounded, and have a maximum length of 10 mm. The ortho- 

 clase fragments are less abundant and have a maximum length of 

 6 cm. These inclusions are scattered through the dikes. They 



^ "tjber die Gliederung des Urgebirges im Spessart," Geogn. Jahreshefte, V (1893), 



lOI. 



= "Die Lamprophyrgange des sudlichen Vorspessart," Neues Jahrb. f. Min., 

 Beilageband VI (1889), 521 ff. 



