14 Prescott — Ilvaite from Shasta Co., California. 



Art. II. — Ilvaite from Shasta Co., California; by Basil 

 Prescott, Stanford University. 



The ilvaite described in this article occurs at Potter Creek, 

 Shasta Co., California. The locality is well known to geolo- 

 gists on the Pacific coast, for it was from the caves in the 

 Carboniferous limestone at this place that Dr. Merriain 

 unearthed the Quaternary vertebrate remains,* and recently 

 it has also attained some economic importance from the 

 exploitation of the magnetite bodies that occur at the contact 

 of this same limestone with an intrusion of diorite. It was 

 during an examination of these ore deposits early in the pres- 

 ent year that the writer noticed the presence of ilvaite. 



Lindgrenf has cited this mineral as a typical product of 

 contact metamorphism, and the occurrence and association as 

 seen in Potter Creek are in accord with this view. There 

 were two occurrences noted. On both sides of a six-inch dike 

 cutting through the limestone, a half -inch band of pure mas- 

 sive ilvaite was found, this, in places, sending out rough rec- 

 tangular prisms into the limestone an inch or more -in length. 

 A few feet away, further search was rewarded by a number 

 of ilvaite crystals associated with a coating of eroded quartz 

 crystals on hedenbergite, a more common contact mineral. 

 The crystals are about 7 to 8 mm in greatest dimension and are 

 well formed, doubly terminated and symmetrical. The extremes 

 in habit are shown in the figures, but even in the more elon- 

 gated the prism zone is not as well developed as in the crys- 

 tals from Elba. 



Although the crystals are bright and untarnished, the sig- 

 nals were not distinct, and close measurements were impossi- 

 ble on account of vicinal faces and striations. The forms 

 present are m (110), s (120), 5(010), 0(111), r{101\ (890) (?) all 

 of which are those more commonly developed in ilvaite with 

 the exception of the doubtful new form (890) found on two 

 crystals, where it replaces the prism m (110). The following 

 measurements serve to identify the forms, the zone [b, s, m,~] 

 taken from one crystal, the zone [0, r,~\ from a second, as the 

 two were not found measurable on the same crystal. 



The most striking physical characteristics of the mineral as 

 seen in this occurrence are the submetallic luster and greenish 

 brown streak. The absence of limonite as an alteration prod- 

 uct is noticeable. The cleavage is not prominent and neither 

 the specific gravity nor the hardness would distinguish it from 



* Sinclair, Cal. Univ. Publ. Am. Arch, and Eth., vol. ii, pp. 1-27, 1904. 

 f Character and genesis of certain contact deposits. Trans. Amer. Inst. 

 M. E., vol. xxxi, p. 227, 1901. 



