230 Reviews — Dr. J. Petersen on Cheviot Roclis. 



received the author's careful attention. It is a leafy fibrous substance 

 polarizing- with a beautiful blue tint and showing marked pleochro- 

 isra — emei'ald green to yellowish green. The polarization is not 

 uniform, the diiferent fibres giving different tints. This mineral 

 is associated with carbonates and small colourless strongly refracting 

 grains which may be regarded as quartz, also M^ith other strongly 

 refracting grains of a yellow colour. 



The green substance gelatinizes with hydrochloric acid. Ex- 

 amined by means of convergent light, the leaves of which it is 

 composed show a bissectrix with a not inconsiderable axial angle. 

 The yellow grains associated with the green substance were carefully'" 

 considered by the author. Precisely similar grains associated with 

 chlorite in a diabase-porphyrite from Bocksteinwald were examined 

 qualitatively by the micro-chemical methods due to Boricky^ and 

 Behrens.^ Lime, iron, alumina and silica were determined, and the 

 conclusion arrived at that they are epidote. 



The chloritic substance was purified as far as possible and analyzed 

 with the following results. 





I. 



II. 



Si02 ... 



... 38-24 



39-98 



A1003 ... 



... 23-15 



19-05 



FeO 



... 12-99 



15-18 



CaO 



6-40 



3-46 



MgO 



8-13 



13-77 



HoO ... 



10-21 



9-14 



NaaO ... 



trace 



— 



99-12 100-58 



The differences in the analyses are attributed to the difficulty of 

 obtaining pure material and to experimental errors due to the small 

 quantity available for analysis — J gramme. 



These results lead to the conclusion that the substance analyzed 

 was a mixture of chlorite, epidote and silica with water. As the 

 chlorite is biaxial, it is regarded as clinochlore. In the altered 

 rocks the base is doubly refracting. It is regarded as having been 

 produced by the devitrification of a glass similar to that of the 

 unaltered rock. 



The base of one of the slightly altered rocks from a point two 

 miles up Allerhope Burn was isolated and analyzed. 



SiOs 65-16 



AI2O3 17-49 



FeoOg 3-01 



Cat) 0-84 



MgO 2-34 



Na.^O 3-68 



K2O 5-54 



H2O 1-76 



99-82 Sp. gr. 2-640 



^ Boricliy, Elemente einer neuen chemisch-mikroskopischen Mineral und Gesteins- 

 analyse. Prag, 1877. 



2 Behrens, Mikrochemische Metboden zur Mineralanalyse. Neues Jahrbuch ; 

 Eef. Band II. 1882. 



