OF ARTS AND SCIENCES. 



305 



'■'■ Epidote Lo(h." Mesnard. — Among the rocks resulting from the 

 alteration of the amygdaloids is one which is generally called epidote, 

 and which seems to have given its name to the " Epidote lode," north 

 of Portage Lake. It is a light-green, hard rock, with an amorphous- 

 looking matrix, and containing amygdules and .seams of quartz. Even 

 the naked eye detects countless miiuite seams of quartz, which trav- 

 erse the matrix in a manner that suggests the former existence of 

 shrinkage-cracks. In thin sections this rock is seen to consist of bris- 

 tling masses of a faintly green mineral, in long, thin prisms. These 

 crystals, when seen in very thin parts of the section, under Hartnack's 

 No. 7, and between crossed nicols, generally become dark when oblique 

 to a nicol plane ; but occasionally one is found in which the maximum 

 extinction occurs when the longer direction of the prism is parallel to 

 a hicol plane ; it is, therefore, a monoclinic mineral. 



These crystals are suspended in quartz, which forms a considerable 

 percentage of the rock, not only as amygdules, but as veinlets and 

 matrix. 



The quartz is undoubtedly the younger mineral, and the appearance 

 suggests that the rock has been an amygdaloid, of which the matrix was 

 altered finally to this green mineral, and then impregnated with quartz. 

 The resemblance to the Huron mine occurrence suggests that the matrix 

 passed through the prehnite stage. 



The absence of free ii-on-oxide in the.^e epidote-quartz rocks can be 

 explained only by supposing that it entered into combination to form 

 the ferric silicate. 



The following is the mean of two analyses made for me, by Mr. 

 G. W. Hawes, in the Laboratory of the Sheffield Scientific School: — 



SiOa 58.87 



AI2O3 13.05 



FeaOg 4.'29 



Feb 2.70 



MnO 20 



CaO 14.88 



MgO 1.60 



K2O 06 



NagO 24 



HoO 4.38 



The free quartz was determined by M. Miiller's 

 method,* digesting the powder with pliosphoric acid. 

 The appearance under the microscope would sug- 

 gest that much more of the silica should be cred- 

 ited to free quartz than the 10.89 per cent given by 

 Miiller's method. I should say that twenty -five to 

 thirty per cent would be nearer the truth. The 

 substance is, probably, quartz, and an epidote very 

 poor in iron. 



100.27 

 Sp. gr. 2.88. Free quartz, 10.89. 

 Sp. gr., after deducting the quartz, 2.91. 



* Journ. f. pr. Chemie, xcv. 43, and xcviii. 14. 



