THE ORIGIN OF THE INCLUSIONS IN DIKES 179 



which is 6 feet in width there are long, narrow inclusions of the 

 country rock, one of which is 15 feet in length and about i foot 

 wide with tapering ends. The thickness of the sedimentary rocks 

 in the region is probably about 4,000 feet, and through this dis- 

 tance the pre-Cambrian inclusions have come. 



Near Ithaca there are about 25 peridotite dikes, some of which 

 contain a large number of shale and limestone inclusions from the 

 country rock (of Upper Devonian age) and from the underlying 

 Paleozoic sediments.' Similarly in the peridotite dikes of Elliott 

 County, Kentucky, there are inclusions of acid (pre-Cambrian?) 

 rocks.^ 



Crazy Mountains, Montana : On the south side of Cottonwood 

 Creek in the Crazy Mountains, Montana, are two elliptical stock- 

 like masses of ouachitite breccia cutting flat-lying Fort Union, 

 Eccene, shales. ^ The masses are 750 feet apart. The lower one 

 is 200 feet long and 40 feet wide, the upper one 300 feet long and 

 200 feet wide. In each case the contacts are vertical and the 

 igneous masses are jointed in both a horizontal and a vertical 

 direction, the joints cutting both inclusions and cement. The 

 fragments consist largely of fine-grained granitic gneiss, feldspathic 

 quartzite, black slate, and granite. The outline of the fragments is- 

 subangular to angular. The interesting feature of these breccias 

 is that the gneissic granite must be of pre-Cambrian age, as no 

 such rock is exposed in the petrographical province. The thick- 

 ness of the sedimentary series underlying the Crazy Mountains'* 

 is 24,000 feet, consisting of 4,500 feet of Algonkian, 3,900 feet of 

 Paleozoic, 100 feet of Juratrias, and 15,500 feet of Cretaceous and 

 Eocene strata. Therefore, the gneiss inclusions have apparently 

 risen vertically for a distance of over 4 miles. 



The origin of the inclusions was probably by thermal combined 

 with mechanical shattering, as the magma has ascended in ellipti- 

 cal intrusions, replacing the country rock. The circulation in the 



^ U.S. Geol. Surv., Geol. Atlas, Folio No. 169, 1909. 



^ J. S. Diller, U.S. Geol. Surv., Bull. 38, 1887. 



3 The writer is indebted to Professor J. E. Wolff for the description of these 

 breccias. 



■t W. H. Weed, U.S. Geol. Surv., Geol. Atlas, Little Belt Mountains Folio No. 56. 



