280 Rerieics — Prof. T. Sterry Hunt — Pre-Camhrian Rocks, etc. 



author alludes, is perhaps more apparent than real. In the first 

 place, it does not follow that, supposing a change from olivine to 

 serpentine to have taken place, the amount of silica should remain 

 constant ; secondly, allowing, for the sake of argument, that the 

 silica underwent no augmentation or diminution, there is every 

 probability of the existence of sufficient of the bisilicate, enstatite, in 

 the original rock to bring up the amount of silica in the resulting 

 serpentine to the normal standard!. It seems, however, that Scheerer, 

 influenced partly by considerations of this sort, was led to reject the 

 notion of the derivation of the serpentine of Snarum, in Norway, 

 from a previously -formed olivine. 



The StratigrapMcal Belations of Serpentine. — But little space is 

 left for us to discuss this section of the subject, which, as treated by 

 the author, involves that of the Arch^an rocks themselves. There 

 is a great amount of information as to Alpine and Italian geology in 

 this portion of the paper, and the numerous references to those who 

 have written on these questions makes it a valuable compendium. 

 Practically there seems to be a fair consensus of opinion amongst 

 most of the recent writers on fundamental points, the old meta- 

 morphic theories as to many of these schists being of Triassic 

 or Jurassic age seeming to be generally abandoned. Thus, Professor 

 Bonney, in his recent lecture at the Eoyal Institution on the " Build- 

 ing of the Alps," gave the following sequence of the presumably 

 Archsean rocks from above downwards : 



C. Lustrous schists. 



B". Friable gneiss. 



B'. Pietre-verdi. 



B. Bedded gneiss. 



A. Central gneiss. 



In this arrangement we have three primary groups, though the 

 central one is made to consist of three members, which, in some 

 eases perhaps, replace each other. 



Comparing this in the same order with the correlations of Dr. 

 Hunt— 



The Lustrous schists =The Taconian. 



The Mica schist series =The Moutalban. 



The Pietre-verdi = The Huronian. 



The central granitoid gneiss = The Laurentian. 



We have already seen that serpentines occur in all three groups 

 above the central granitoid gneiss, but are especially abundant in the 

 Pietre-verdi, or zone of greenstones. Since the crystalline rocks of 

 Italy, according to the majority of writei'S quoted by the author, are 

 merely the Alps deflected and brought up again, it follows that the 

 Italian serpentines occupy for the most part positions analogous to 

 those in the Alps. 



This is known and admitted in many cases to be so, but Dr. Hunt 

 will not hear of any Tertiary serpentines at all. Even the celebrated 

 mass of Monteferrato, in Tuscany, which Bonney maintains (Geol. 

 Mag. Aug. 1879) to be of late Cretaceous or early Tertiary age, is 

 set down as Archaean, just as are certain serpentines in connexion 

 with Triassic strata near the city of New York, which are described 



