Epidote as Rock-forming Minerals. 225 



While engaged in the study of the geology of a section of the 

 Johns Hopkins University map, two epidotic minerals were 

 found to play an interesting role as accessory constituents of 

 the porphyrinic granite of Ilchester. A short notice on the 

 geological relations of the rocks of this area, together with 

 mention of parallel growths of epidote and allanite, appeared 

 in April, 1888.* After a further examination of these inter- 

 esting intergrowths, it is my purpose in the present paper to 

 consider them more fully in connection with what has already 

 been said concerning epidote and allanite. 



The Ilchester granite, in which they occur, is one of the 

 youngest of a series of eruptions in the gneiss and crystalline 

 schist of eastern Maryland. It is a medium to coarse-grained 

 rock, with a porphyritic aspect caused by the large microcline 

 crystals scattered through the holo-crystalline to granophyric 

 ground-mass. This ground-mass contains as essential constit- 

 uents, varying proportions of monoclinic and triclinic feld- 

 spar, and biotite By the more or less complete replacement of 

 biotite by hornblende, the granitite develops facies of horn- 

 blende granite. Besides allanite and epidote the only impor- 

 tant accessory constituent is a colorless mica. Throughout the 

 entire area of the section studied, the granite shows evidence 

 of " stretching" in its more or less perfect " parallel structure," 

 and in the broken character and disturbed optical properties of 

 the constituent minerals. 



The epidote is macroscopically visible in the rock, generally 

 as yellow columnar crystals one. to three millimeters in length, 

 though it is often without crystalline form. A brownish ker- 

 nel is frequently visible within the epidote, and when prismatic 

 planes are developed upon the latter, the perfect parallelism 

 of the corresponding faces of the two minerals can be observed. 

 The junction of the included mineral with its host appears as 

 a sharp line, owing to the difference in color of the two 

 minerals. The included mineral shows no distinct cleavage, 

 is very brittle, and is found in some specimens of the granite 

 decomposed to a brown powder. The brittle character of this 

 mineral has prevented its removal from the matrix and ex- 

 amination with the goniometer. 



Under the microscope its isomorphous relation to epidote is 

 strikingly shown in many instances by the parallelism of the 

 bounding planes of the two minerals and its distinct character 



* Wm. H. Hobbs, On tbe rocks occurring in the neighborhood of Ilchester, 

 Howard County, Maryland ; being the detailed study of the area comprised in 

 sheet No. 16 of the Johns Hopkins University map. Johns Hopkins University 

 Circulars No. 65. (Preliminary notice of a dissertation for the degree of Doctor 

 of Philosophy). 



