PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 231 



Amygdaloid. ] 



probably a portion of the glassy magma, and the present contents are the result of 

 change from the magma. They have the cloudy or cumulus aspect of the minutely 

 polarizing substance No. 2, in No. 140(7), and the hardness is about the same. They 

 also have the minutely fibrous structure, arranged perpendicularly to the clouded 

 banding, with extinction parallel with the fibres which are positive in elongation. 



Three sections. 



Remarks. There is a considerable quantity of this substance in the slide. 

 Sometimes it forms a coating about the zeolitic nests, and is then minutely fibrous, 

 as above described. In other cases it is massive, and completely fills some of the 

 larger cavities. This cannot be affiliated easily with the brown mineral above 

 mentioned as probably bowlingite, yet, according to the distinctions that have been 

 made by Heddle and Lacroix, they are probably the same substance, and may have 

 resulted from the alteration of olivine,* with contributions from the other minerals. 



According to Michel Levy olivine crystals in the basalts of Auvergne are some- 

 times peripherally and sometimes centrally transformed into a reddish-brown, highly 

 refractive and doubly refractive mineral, having distinctly different optic properties 

 from olivine, which, according to Lacroix, is goethite. This mineral is easily distin- 

 guishable from that under consideration, although they frequently have the same 

 color. Bowlingite is less refractive and goethite more refractive than olivine. 



In many of the diabases of the north shore of lake Superior olivine has changed 

 partly into a similar brown mineral, first noted in No. 133, and partly into a finely 

 fibrous, colorless mineral. In No. 162 both these conditions are found in the same 

 changed olivine grain, the ferruginous brown mineral occupying the periphery, and 

 the finely fibrous mineral the central area. In one non-fibrous grain a distinct cleav- 

 age is visible and can be traced from the brown into the colorless portion, indicating 

 that the coloration is an accidental character. In some cases the central portion 

 is not a clearly cleavable mineral, but a fibrous or a minutely scaly one. When it 

 is fibrous its elongation is positive, but these fibres are in the midst of less fibrous 

 and even of non-fibrous conditions of the same mineral. These conditions appear to 

 belong to the same substance. Olivine is well known to be a favorite gathering place 

 for magnetite whenever those changes take place which provoke its accumulation. 

 In this case the ferruginous element seems to play the part simply of a coloring 

 agent, and to fade out toward the centre of the original grain. It is highly probable 

 that if it were abundant it would manifest distinctly the characters of goethite, as 



*On this subject the student is referred to the following authorities: 



HANNAV. Mni<'r<'i<.ii>-'il Magazine, vol. i,p. 154, 1877. 



HEDDM!. Transactions of the Royal Society, Edinburgh, vol. xxix, p. 91, 1879. 



LACBOTX. Bulletin de la SociUe cle Minfralogiede Frame, vol. viii, p. 97, 1885. 



MICHEL LKVY. BullHin de la Socieli Geologi'juc tie France, xviii, p. 831, 1890. 



DANA. System of Mineralogy, 1892, p. 682. 



IDDINGS. Geology of the Eureka District, U. S. Geol. Survey, Monograph, xx, p. 387, et seq. 



LAWSON. Bulletin of the University California, vol. i, p. 31, 1893. 



LACROIX. Mintralogie de France, vol. i, pp. 174, 442, 1895. 



