Cincinnati Society of Natural History. 



Rowlandville granite in its original condition. By far the 

 most important of these new minerals is cpidote, which has 

 been formed on a very extensive scale. While this mineral 

 occurs in nearly all the constituents, it is most abundant and 

 most typical in the feldspar. 



Since the epidote is abundant in feldspar crystals, which 

 otherwise show no change, and in fresh granites where there 

 has been no mineral decomposition by atmospheric agencies, 

 its origin must be regarded as metamorphic rather than as 

 due to weathering, although this has been frequently asserted 

 to be the case.* The replacement of feldspar by epidote in 

 the Rowlandville granites varies from nearly complete pseudo- 

 morphism to cases where only a slight amount of the new 

 mineral is developed. In crystals of the latter sort the 

 progress of change may be traced. It is then seen that the 

 small epidote particles are equally disturbed through the 

 feldspar, indicating that the alteration originates with equal 

 readiness at any point. Where there is the least epidote it 

 forms small crystals with sharp boundaries, and the character- 

 istic monoclinic habit. As the alteration progresses, these 

 small crystals unite into larger masses, which are easily seen 

 under the low powers of the microscope to be a collection of 

 separate grains of pale yellow color, often pleochroic. This 

 change may, in certain cases, go so far as to conceal the 

 original feldspar crystal, thus forming an incomplete epidote 

 pseudomorph after feldspar. It is incomplete, because the 

 outer border of the feldspar is always free from epidote. 

 There is no especial tendency of the epidote to develop in or 

 near cracks in the feldspar, nor does its arrangement bear 

 any relation to the cleavage lines or twinning striae of the 

 latter. Thus, the host apparently exerts no orienting influence 

 on the epidote, as is often stated to be the case. Moreover, 

 in feldspar crystals which show pressure effects, such as 

 undulatory extinction, a development of microcline structure, 

 or a peripheral granulation of the crystal, little or no epidote 

 is formed. This accords with the law of reciprocal relation 

 between the amount of chemical alteration and the amount 

 of crushing stated by Williamsf in his work on the Michigan 

 green stones. 



" Vide Literature references in another place. 

 fBul. U. S. Geol. Survey 62, p. 88, 1890. 



