^{page 124 note 1} It used to be considered that in sheaves and spherulites of this description all negative fibres were felspar and all positive fibres were quartz, but it has been demonstrated that this is not necessarily so, as felspar fibres may also be positive, though it is more rare. So that though negative fibres are felspar only, positive fibres may be either felspar or quartz, and the relative degree of bi-refraction must assist in distinguishing as far as possible. (See full resumé of this subject in Zirkel's Petrographie, vol. i. 1893, pp. 474 and 476.)

^{page 126 note 1} A part is present as ferrous oxide—hence excess in the analysis.

^{page 129 note 1} High authorities, in reviewing all the evidence, have had to come to the conclusion that such transfer takes place at basic contacts; as, for example, Zirkel in his detailed consideration of the contact-effects of diabases (Petrographie, vol. ii. 1894), and also Roth (Chemische Geologie, vol. iii. 1893). No geologist has made such a special study of the chemical side of petrology as Roth, and a great amount of the evidence on this particular question is collected and summarized in the volume named. In reviewing this evidence Roth says that, according to it, “there is a decided contrast between the contact-effects of granite, syenite, etc., and those of diabase: in the former case no introduction of material into the altered slates and no chemical differences due to distance from the eruptive rock; whereas in contact with diabase we have introduction of silica and soda into the altered slate, and at the same time chemical difference according to distance from the eruptive rock.”