306 C. It. KEYES — EPIDOTE IN ERUPTIVE ROCKS. 



erals in the Maryland acid rocks was firsl noted by Hobbs,* who had 

 under consideration the porphyritic granites of the last two localities just 



mentioned. In the published notes particular attention is called to the 

 isomorphous intergrowths of epidote and allanite. Since the appearance 

 of the paper alluded to considerable additional materia] has been ex- 

 amined, both from the Ilchester and Ellicott City districts, besides three 

 other places. 



Comparison of Granites with Granitites. — In all their general characters 

 the epidote-allanite bearing granites are essentially identical with the 

 granitites of the region, except as a rule they are more basic and conse- 

 quently much darker in color. The essential constituents show no note- 

 worthy differences from the acid components of the typical granitites. 

 In hand specimens a distinct greenish cast is often quite noticeable. Upon 

 closer investigation the green specks are f< >und to have frequently a reddish 

 core. Microscopic examination in thin sections show that the two min- 

 erals are clear, usually idiomorphic or hypidiomorphic epidote. and 

 reddish, intensely pleochroic allanite in parallel growth-. 



Allanite. 



The mineralogic Associate of Epidote. — Ik-fore considering the epidote in 

 detail a few words in regard to the allanite may not he out of place, as it 

 is intimately associated and closely related chemically. 



As a rock-forming Mineral reviewed. — As a rock-forming mineral, allanite 

 has long been regarded as one of the rarer occurrences. Within the pasi 

 few years, however, hidings and Cross f have found this silicate of the 

 rare earths widely distributed among acid eruptives, in some rocks form- 

 ing an important accessory. Among the rocks in which the mineral 

 under consideration was found may he mentioned gneiss, granite, quartz- 

 porphyry, rhyolite, diorite, porphyrite, andesite, dacite and others. The 

 localities in this country where allanite has been found to form a rock 

 constituent are numerous, and are even widely separated geographically. 



In Europe the apparent rarity of rock forming allanite has made the 

 observed occurrences somewhat noteworthy. There is a further interest 

 centering around this mineral which is of no little importance from a 

 historical point of view also. It is the fact that the presence of allanite 

 in granite formed one of the chief arguments againsl the theory of the 

 igneous origin of granite in the long-continued controversy that took 

 place during the second quarter of the present century. The inability of 

 this mineral to withstand a temperature higher than a dull-red heat 



* Johns Hopkins University Circulars, no. 85, 1888, i>. 70, 

 fAm. Jour. Sci . 3d ser., vol. sxx, L885, p. L08, 



