BY W. N. BENSON. 687 



of translucent, finely divided prehnite developing at the expense 

 of the colourless garnet, which forms the ground-mass. 



This garnet-rock is, doubtless, that described by Professor 

 Judd(42) as forming a vein near Bingara; and which has been com- 

 pared by Professor Marshall to the grossularite-diallage rock from 

 the Dun Mountain, New Zealand, which lie has termed rodin- 

 gite(43). The comparative study of some of his material, as well 

 as chemical analysis, shows the correctness of this identification, 

 but I cannot concur in Professor Marshall's views as to the origin 

 of this rock. Abandoning his former view, that they were gabbros 

 modified by absorption of limestone!**), he now considers them 

 to have crystallised out in their present state; and he compares 

 them with the ariegite group of garnet-peridotites. I have 

 studied Professor Lacroix's type-collection of ariegites, and cannot 

 see that they resemble rodingite at all. The long series of 

 alterations of eucrite recorded above, and the regular increase in 

 specific gravities, show clearly that the grossularite-rock is an 

 extremely altered form of eucrite. It often occurs with prehnite, 

 as seen above, and as recorded by Marshall. The Bingara rodin- 

 gite occurs among saussuritic eucrites, but, until field-evidence 

 has been more fully studied, I cannot suggest how they have 

 become so altered. It was certainly not by absorption of lime- 

 stone. 



A final and entirely different manner of alteration is shown by 

 M.B., 51, from Upper Bingara. In hand-specimens, it appears 

 to be a gabbroid rock that has been highly sheared and veined. 

 Mineralogically, it is altered beyond recognition as a gabbro. It 

 consists chiefly of tremolite aggregated in ragged, multiply- 

 twinned plates. These have a rough parallelism with a single 

 shearing direction, but are locally contorted and interwoven. 

 They are set in a ground-mass of clear albite-felspar, occasionally 

 showing bent, multiple twinning. The rest of the rock is made 

 up of large veins of prehnite. 



The various changes in the gabbros, that have been described, 

 are usually those considered as taking place under pressure. 

 Besides the very frequent distortion which the altered rocks 

 have suffered, an interesting confirmation is obtained by the 



