i8o SIDNEY POWERS 



magma must have been very active to bring up blocks of gneiss 

 from sjLich a depth. 



Beemersville, New Jersey: Near Beemersville and Libertyville, 

 New Jersey, are several masses of breccia whose form suggests 

 narrow volcanic necks or stocks. The occurrences have been 

 described by Kemp^ and by Wolff .^ 



The fragments are of various sizes from bowlders to fine grains 

 and consist of gneiss, granite, limestone, and shale (the country 

 rock), imbedded in an ouachitite. The shale inclusions show very 

 sharp angular edges. The presence of inclusions of gneiss show 

 that there has been vertical transportation of nearly a mile. Here 

 again the elliptical-shaped intrusions have probably come up by 

 replacing the rock through which they have passed, and the inclu- 

 sions probably represent the stoped blocks. 



There are also a number of lamprophyric dikes in the region 

 which contain abundant inclusions of the rocks which they cut. 



SUMMARY 



Inclusions in dikes are rare and are due to special or to acci- 

 dental causes. The majority of the inclusions have been shattered 

 from the walls of the dike either in the formation of the fissure 

 through which the dike came or in the injection of the dike-magma. 

 The shattering is largely a mechanical operation, but thermal and 

 mechanical action may be combined in many cases. In a few 

 instances the inclusions are pebbles derived from a conglomerate. 



The direction and amount of the movement of the inclusions 

 in the several examples are shown in the table on p. i8i. 



In the cases where some inclusions rise and others sink in the 

 same dike, as at LaTrappe near Montreal, the movement does 

 not appear to depend as much upon the specific gravity of the 

 magma relative to that of the inclusions as upon the mechanics of 

 intrusion and the circulation in the dike-magma before its con- 

 solidation. Sandstone fragments have descended in this dike 

 although their specific gravity when in the magma was probably 



'"On Certain Porphyritic Bosses in Northern New Jersey," Amer. Jour. Sci. 

 (3), XXXVIII (1889), 130-34. 



^ U.S. Geol. Surv., Geol. Atlas, Franklin Furnace Folio No. 161, p. 13. 



