Professor S. J. Shand — Saturation in Petrography. 339 



have been made. It may have consisted of lichens, the hard fibrous 

 stems of heath-like plants, or, perhaps, tough bark and woody tissue. 

 Why the food should have been thus restricted is not clear; the 

 climate cannot have been very cold, for the remains of a large land 

 tortoise {Testudo gymnesica, Bate) have been found in the same 

 deposits. On the other hand, there may have been conditions of 

 aridity which led to the stunting of the vegetation. In any case 

 the food must have been widely different from that of other members 

 of the group in which" no trace of such a modification of the dentition 

 has occurred. 



The skeleton has been mounted with great skill and care by 

 Mr. L. T. Parsons, jun. Some approximate dimensions of the 

 specimens as mounted (see Plate XII) are: — 



cm. inches. 



Greatest length 88 34£ 



.Height to tip of horns .... 63 24§ 



Height to highest point of back . . 49 19| 



Height to top of scapula . . . 44j 17^ 



Height to top of pelvis .... 43 17 



II. — The Principle of Saturation' in Petrography : A Ueplt. 



By Professor S. J. Shand, D.Sc, F.G.S., Victoria College, Stellenbosch, 



South Africa. 



IN exercising a debater's right to reply to criticism, I shall be 

 extremely brief. My contentions, in two essays on the above 

 subject, 1 were as follows : — 



1. That the classification of rocks by silica percentage is unsatis- 

 factory, and that a more natural basis for classification is available in 

 the degree of saturation of the constituent minerals. 



My critic, Mr. Scott, has not expressed any opinion on this point 

 of comparison. 



2. That the field evidence is overwhelmingly favourable to the 

 separation of the rock minerals into two groups, those of one group 

 being stable in the presence of free silica under magmatic conditions, 

 those of the other group unstable ; and that such experimental work 

 as has been done supports that separation. 



Mr. Scott has not brought forward a single fact in refutation of 

 this, but only a set of assumptions, considerations, probabilities, 

 possibilities, and plausibilities. (I wonder if he realizes how much 

 of his two communications consists of statements such as "this might 

 happen", "the reaction may be appreciably reversible", " meta- 

 silicates might dissociate", "if, as is probable, fayalite dissociates ", 

 "probable origin of garnets", "probable instability of pyrope ", "if 

 it had crystallized ", and so on ?) 



The recent work of 0. Anderson 2 on the system anorthite-forsterite- 

 silica shows afresh the instability of the iorsterite-silica association. 

 It is claimed, however, that " corroded crystals of olivine may be 

 imbedded in reaction products or other minerals in such a way that 

 they are protected against further reaction with the magma. In this 



1 Geol. Mag., November, 1913, p. 508, and November, 1914, p. 485. 



2 Amer. Joum. Sci., April, 1915. 



