Lawson.] 



Geology of Carmclo Bay. 



33 



In deference to these views, and in order to facilitate comparisons, 

 it will be convenient to give its crystallographic axes a position in 

 space corresponding to that of olivine. Now, if we consider the form 

 of the most common cross sections exhibited in the slides, we find 

 that there are two pronounced types, viz.: (i) A set of cross sec- 

 tions which may be easily recognized as parallel to the plane of 

 eminent cleavage. These are represented by Figs. 2, 3, 4, 5. 

 Plate 4. (2) A set of cross sections which are transverse to the 

 cleavage, Figs. 6, 7, 8, Plate 4. The latter correspond in form 

 to the basal sections of olivine, while the former correspond to 

 a pinacoidal section. In the basal sections, moreover, the cleav- 

 age traces subtend the large prismatic angle 110 ^ 110=130°, 

 and is therefore in the plane of the macropinacoid. We have thus 

 the following for our crystallographic scheme: ]> in the cleavage 

 plane, a perpendicular to the cleavage, and j in the cleavage and 

 parallel to elongation of the crystals. This statement that the 

 cleavage corresponds to the macropinacoid (100) is at variance with 

 that of Michel-Levy and Lacroix, who affirm that the cleavage of 

 the decomposition product to which they refer is parallel to the 

 brachypinacoid (010).* They also state that the optic axf; appear^ 

 to be situated in the plane of the cleavage. In iddingsite 4*- is nor- 

 mal to the cleavage. flu/ftf** 



The orthorhombic symmetry of iddingsite, which is assumed in 

 the above comparison with olivine, is proved by its optical charac- 

 ter. Under the microscope the cleavage plates prove to be biaxial 

 and yield with great definiteness an interference figure which shows 

 that the plane of the optic axis is at right angles to the cleavage, 



(2) Zirkel F., Basalt gesteine Bonn, 1870, p. 65, also Microscopical Petro- 

 graphy of the 40th Parallel, Wash., 1876, p. 230. 



(3) Rosenbusch, Mikroskopische Physiographic, Stuttgart, 1877, p. 400. 



(4) Michel-Levy et Lacroix, Les Mineraux des Roches, Paris, 1888, p. 248: 

 "L'olivine se transforme en serpentine, ou en un corps ferrugineux rouge par 

 transparence, a clivage facile suivant g' (010) dont les axes optiques paraissent 

 etre situes, ;i l'inverse du peridot dans un plan de la zone d'allongement. 

 Parfois ce produit d'alteration est polychroique." 



*Loc. cit. The position of the axis of olivine in the Mineraux des Roches is 

 not the same as that in German, English, and American works, a and c being 

 transposed. But the adjustment of this difference of usage would not reconcile 

 the two statements. 3 



