SUMMARY. 



311 



minerals were formed under the same .physical conditions, so that any 

 remarks upon the origin of the one would apply equally as well to the 

 other. In attempting to determine whether or not these minerals are of 

 primary or secondary origin in granitic rocks, the evidence must neces- 

 sarily be based in great measure upon the observed association with the 

 other minerals. Allanite, as has already been stated, is comparatively 

 easily fusible, and on this account it long has been quoted as one of the 

 proofs against the igneous origin of granite; but its occurrence in such 

 rocks as unaltered porphyrite, quartz-porphyry, dacite, andesite, and 

 rhyolite masses whose eruptive nature, as shown by hidings and Cross* 

 is not to be questioned, shows conclusively that this mineral actually 

 does form in a molten magma. 



Furthermore, the epidote occurs included in well-defined crystals of 

 sphene whose primary character cannot be doubted; besides, it is not 

 uncommon to find sharply denned crystals completely mantled by bio- 

 tite, along with similar inclusions of zircon, apatite and magnetite. 

 There is further evidence pointing toward the original character of the 

 epidote in the occurrence of broken crystals of allanite-epidote inter- 

 growths, into the open fractures of which biotite has formed. To all 

 appearances these fractures are protoclastic in nature. Finally, crystals 

 of epidote or isomorphous growths of epidote and allanite, with the 

 crystallographic planes well defined, are found giving shape to the unal- 

 tered feldspars, quartz ami mica. 



Summary. 



In summing up the facts already presented it would appear that the 

 evidence of the primary occurrence of epidote in the eruptive rocks is 

 essentially the same as that for allanite. Attention has been called 

 to the fact that allanite. though easily fusible, is now known to be widely 

 distributed, and is often an abundant accessory in such rocks as dacite, 

 porphyrite, diorite, quartz-porphyry, rhyolite and others the igneous 

 nature of which cannot be questioned. All physical obstacles as to its 

 primary origin are manifestly removed. The evidence, therefore, in any 

 particular case that this mineral is either primary or secondary must be 

 derived largely from the study of its associations with other minerals. 



Now the epidote of certain of the Maryland granites is found in 

 isomorphous growths with allanite, as well as in separate well-defined 

 crystals. Both occurrences are found in sharply bounded individuals, 

 and the following remarks apply to the intergrowths and single crystals 



* Am. Jour. Sei. 3d ser., vol. xxx, 1889, p. 109. 



