FENNER, THE WATCHLNG BASALT 139 



color, which, under the highest powers, has a granular appearance. This 

 opaque substance is not identifiable with certainty but is evidently some 

 material which could not be worked over or digested in the process of 

 recrystallization and therefore remained as an insoluble residue. It 

 would appear that either TiO, or MnO, might act in this manner. Both 

 are shown by analysis to be present in the basalt, and the opaque, brown- 

 ish dust generally present in the garnet and at times in other secondary 

 minerals may be referred with great probability to one or both of these 

 oxides. The relief of the crystal grains is very high. The Becke test 

 indicates much higher index than adjacent prehnite and therefore higher 

 than 1.65. The characteristic outline is that of a regular haxagon. On 

 account of the high relief, it is sometimes possible with good magnifica- 

 tion to make out a number of faces of an individual grain and to deter- 

 mine that the form is that of a rhombic dodecahedron (110). In most 

 cases, the grains appear isotropic, but occasionally a very low birefringence 

 is perceptible. The crystalline form, high relief, habit of including for- 

 eign material, lack of cleavage and anomalous birefringence correspond 

 with garnet. This mineral has not heretofore been recognized under 

 exactly similar circumstances, so far as the writer is aware. 



The hexagonal outline of the garnet is best preserved where the growth 

 of the associated prehnite has made least advance. Where the prehnite 

 reaches a more notable amount, the garnet gTains become rounded, and. 

 with good magnification, the surface appears pitted. At a further stage, 

 the grains become smaller and finally disappear, leaving the brown dust 

 remaining as a turbidity in the prehnite. There is some evidence that the 

 garnet is likewise yielding to the advance of the green amphibole. The 

 inference is that the period of deposition or field of stability for the 

 garnet was very limited. 



In slide 53 (Plate XI, fig. 5), an original glassy crust on dense basalt 

 has been metamorphosed to a mixture of garnet, green amphibole and 

 specularite, which has subsequently been attacked by prehnite. The in- 

 soluble residue of the original glass (TiO, or MnOo) has been included as 

 before as a muddy brown sediment in the garnet grains, giving to many 

 of them an opaque appearance. By reflected light, this has a milky look. 

 The spherulitic markings and curved bands, which have been spoken of 

 before and which are illustrated in the figure, are probably surviving 

 structures of the original glass. The garnet grains are individually 

 rather larger than those in 98 (about 0.0T5 mm.) and in places are so 

 crowded as to form considerable masses, some of which are perfectly 

 opaque from undissolved residue. Where invasion by prehnite has pro- 

 gressed, the garnets are much corroded. Slide 100 is from the same hand 



