THE ORIGIN OF THE INCLUSIONS IN DIKES 171 



A narrow-necked embayment, 3 inches in depth in the end of an 

 inclusion i foot in length and 3 inches in width, has been noted by 

 Jaggar. The quartz inclusions all appear to be surrounded by a 

 wreath of augite prisms about i mm. wide, characteristic of quartz 

 inclusions in basalt. This endomorphic reaction rim was found by 

 Jaggar to consist of four zones between the diabase and the quartz : 

 (i) diabase feldspar, (2) augite crystals, (3) potash feldspars, (4) 

 micropegmatite. 



The inclusions in these dikes appear to have been principally 

 derived from rocks underlying the Cambridge slate. Very coarse 

 diorite pegmatites, specimens of which have been found at Kidder 

 Avenue in the Medford dike, and quartz veins seem to have fur- 

 nished most of the material for the inclusions, the fragments of rocks 

 being quite scarce, as would be expected in normal dikes shattering 

 a few blocks off their walls during their ascent. The diorite peg- 

 matite is not exposed in the vicinity of the dikes unless it is the 

 same as that described by Jaggar near Arlington Heights. 



These inclusions may have floated up in the dike-magma or 

 they may have been forced up by it. Day et al. {op. cit.) have 

 shown that the density of diabase glass is 2.763 and therefore 

 heavier than the mineral xenocrysts. Whether the latter were 

 formed by the differential expansion of the various minerals, a theory 

 whose importance is emphasized by Goldschmidt,^ or whether they 

 were formed by purely mechanical action, is not clear. Numerous 

 small xenocrysts of quartz and feldspar, from the vein quartz and 

 diorite pegmatite, are found throughout the diabase, at least part 

 of which have been broken off the corners of the inclusions. 



Quartzose inclusions appear to be corroded frequently in igneous 

 masses. A number of cases of reaction rims around quartz inclu- 

 sions in extrusive rocks will be found described by Lacroix.^ In 

 the Globe district, Arizona, intrusive diabase contains numerous 

 inclusions of vein quartz which are conspicuously corroded and 

 embayed and surrounded by reaction rims of amphibole.^ The 



^ Die Kontaktmelamorphose im Kristiamagehiet (191 1), pp. 107-9. 

 ^ "Les enclaves des roches volcanique," Annates de I'Academie de Macon, X (1893) . 

 3 U.S. Geol. Surv., Geol. Atlas, Globe Folio (No. 11 1), 1904. Other diabase intru- 

 sions are described which contain many inclusions which have sunk. 



