Epidote as Rock-forming Minerals. 227 



These characters belong to the mineral allanite and a compari- 

 son shows that they are the same as are possessed by the 

 allanite of the Ten Mile Region of Colorado.* Further com- 

 parison with slides from the epidote-gneiss of Wermland in 

 Professor Rosenbusch's collection shows that in this case, as in 

 the Ilchester granite, we have to do with parallel intergrowths 

 of allanite in epidote. 



Since publishing my first notice of these intergrowths, the 

 suggestion has been made that the surrounding mineral may 

 not be epidote, but that it may be allanite whose color and 

 optical differences are to be accounted for by slight differences 

 of chemical composition ; in other words that we have to do 

 with a case of zonal structure in allanite. The characters of 

 the epidote require therefore careful consideration. 



The examination in thin section under the microscope shows 

 the epidote to possess, in the majority of cases, the prismatic 

 planes, M, r, and T. The longer sections of these crystals 

 exhibit cleavage lines parallel to the longer axis, and yield 

 in converging polarized light an optic axis, which often 

 appears on the edge of the field. The plane of the optic axes 

 is perpendicular to the ortho-diagonal and the optical angle is 

 large. The cross-sections of crystals show an imperfect cleav- 

 age parallel to M (001), and a poor cleavage parallel to T (100). 

 The cleavage angle measured 115°. The angle of maximum 

 elasticity lies in the acute angle /3 and makes with the vertical 

 axis an angle of 3° to 3° 3'. The pleochroism is distinct and 

 as follows : a nearly colorless, b light straw-yellow, c siskin- 

 green The absorption is written c>b > a. 



By the use of the Thoulet solution a quantity of epidote 

 together with some admixed allanite and titanite was separated 

 from the rock, and "45 gram obtained pure by the tedious pro- 

 cess of picking out the fragments of allanite and titanite, their 

 darker brown color serving to distinguish them. I am in- 

 debted to the IT. S. Geological Survey for an analysis of this 

 powder by Dr. W. F. Hillebrand. Special examination was 

 made to determine the presence or absence of the rare earths, 

 but no trace of any was discovered. The material used in this 

 examination necessitated a determination of the alumina by 

 difference. The amount of ferrous oxide was not determined. 

 The water was determined by loss on glowing. The results of 

 Dr. Hillebrand's analysis are given under I. An analysis of 

 the Untersulzbach epidote by Ludwig is introduced under II 

 by way of comparison. 



* The sections of the biotite-porphyrite of this region were kindly loaned me 

 by Dr. Cross of the U. S. Geol. Survey. 



