622 /. H. L. VOGT 



According to the investigations made at the Geophysical Labora- 

 tory in Washington, the inversion point between pure a = CaSi03 

 {pseudowollastonite) and /3=CaSi03 {wollastonite) has been stated 

 at about 1200°. Some amount of MgSiOj entering isomorphously 

 into the silicate makes the inversion point rise; thus about 6 per 

 cent MgSiOj brings about a rise up to 1300° and 17 per cent 

 CaMgSizOe { = ca. 8 per cent MgSiOj) even up to 1345=^10° C.^ 

 The inversion point of the mineral wollastonite occurring in the 

 contact zones, containing as as general rule a little MgSiOj, must, 

 accordingly, be estimated, at the pressure of one atmosphere, at 

 about 1250-1300°, and at a high pressure even a little higher 

 temperature must be presumed, though, on account of the small 

 difference of density between the two forms, the rise will probably 

 be quite inconsiderable.^ 



As will be brought out in a later paper, the eruption of the 

 magmas, as a general rule, took place at a temperature which was 

 for the deep-seated rocks almost exactly identical with the tempera- 

 ture of the beginning crystallization. For the porphyry rocks we 

 may in many cases assume a temperature of the eruption even 

 somewhat below that of the beginning crystallization. 



If we leave out of consideration the peridotites, anorthosites, 

 and analogous anchi-monomineral deep-seated rocks, the eruption 

 temperature, accordingly, must only exceptionally have been so 

 high as about 1300°. For many anchi-eutectic rocks, I estimate it 

 at about 1250°, and for granitic rocks, even much lower still, as 

 1000° or somewhat less. Thus, it is easily explained that in the 

 contact zones there always occurs wollastonite and never pseudo- 

 wollastonite. 



MgSiOy — ^According to the investigations made by O. Andersen 

 and N. L. Bowen^ MgSiOz has, at usual pressure, no true melting 

 point, as, at 1557°, MgSiOz is divided into solid Mg2Si04 (forsterite) 

 and liquid. The weight proportion of the two phases is 5.5 per cent 



'■ T. B. Ferguson and H. E. Merwin, Amer. Jour, of ScL, XL VIII (1919), p. 165, 

 and the earlier investigations here cited. 



'White and Larsen, Amer. Jour, of Sci., XXVII (1909), p. 421; cf. V. M. Gold- 

 schmidt, "Die Kontaktmetamorphose im Kristianiagebiet" (191 1), p. no. 



i Amer. Jour, of Sci., XXXVII (1914), and Zeiis.f. anorg., Ch. 87 (1914). 



