Vol. 5] 



Louderback. — Benitoite. 



369 



stringers, as judged in the outcrops, appear to contain nothing but 

 natrolite and then play out and disappear. 



Going westward the proportion of neptunite to benitoite in- 

 creases until the latter practically disappears before the stringers 

 die out in that direction. 



THE EELATION TO THE SEBPENTINE. 



It would be interesting to determine whether the serpentine 

 intrusion has had any direct effect on the production of the 

 phenomena described. General considerations seem to the writer 

 to indicate that it had. 



The most suggestive characters of the deposit in this connec- 

 tion are perhaps the similarity in nature of the general meta- 

 morphism of the enclosing rock-lens to the more active alteration 

 in immediate proximity to the vein on the one hand; and on the 

 other hand the correspondence of the albitic alteration, the albite 

 stringers of the country, the albite of the thin zone, and the 

 natrolite in the benitoite veins which are all related chemically 

 and by field relations. In other words, the general rock changes 

 which seem to demand a general elevated temperature for their 

 consummation, appear genetically related forerunners of the 

 local action along the veins. Such a general elevation of tempera- 

 ture was undoubtedly supplied by the peridotite intrusion, and 

 that occasion is most naturally taken as the period of meta- - 

 morphism. 



As regards the more extreme metasomatosis and later miner- 

 alization, it would seem that the rocks of the included mass were 

 more favorable to the production and maintenance of channels 

 than the surrounding peridotite, and acted as a vent pipe for the 

 escape of solutions from the more highly heated lower portions of 

 the peridotite mass to the overlying rocks, or possibly to the 

 surface. 



In considering the possibility of an origin of the deposit pre- 

 vious to the peridotite intrusion, the close relationship of the 

 distribution of the veins and the present form of the included 

 mass is suggestive. The veins run along in the direction of 

 elongation of the lens and become smaller, less mineral-bearing 

 and play out just before reaching the serpentine in either 

 direction. 



