PETROLOGY IN AMERICA. 477 



not a pseudomorph after olivine. Without prejudging this 

 question, it may be remarked that numerous British basalts 

 and diabases contain a mineral agreeing pretty closely with 

 the description of iddingsite, but undoubtedly derived by 

 alteration from olivine, unchanged relics of which sometimes 

 remain in the heart of the pseudomorph. 



Palache (30) has described a rock occurring as a lava- 

 flow of late Cretaceous or probably early Tertiary age in 

 the Contra Costa hills, north of Berkeley. It is an acid 

 lava in which soda preponderates largely over potash, and 

 it is accordingly styled soda-rhyolite. It shows gradations 

 from a porphyritic variety with microcrystalline ground- 

 mass to a pure glass, the bulk of the flow having a 

 microspherulitic structure. One variety consists largely of 

 spheroidal bodies up to three inches in diameter, hollow 

 and containing chalcedony, etc. : these are probably altered 

 large spherulites, and the author's account of them reminds 

 us of those so frequently met with in the ancient rhyolitic 

 lavas of North Wales and other districts. 



The same geologist has investigated a blue soda- 

 amphibole which seems to have a wide distribution among 

 the crystalline schists of the Coast Ranges of California 

 (31). The rock in which the mineral was specially studied 

 consists of this mineral and albite. The mineral has the 

 usual crystallographic habit and cleavages of a hornblende. 

 It is strongly pleochroic in blue, violet, and yellowish-brown 

 colours, and its extinction-angle is about 13 . It bears a 

 considerable resemblance to riebeckite, and a chemical 

 analysis places it between that mineral and glaucophane. 

 The author names the new variety crossite, as apparently 

 identical with a blue amphibole described by Whitman 

 Cross in Custer County, Colorado, and one more soda- 

 bearing amphibole is thus added to the number already 

 recognised. 



Another new rock-forming mineral has been described 

 by Ransome and Palache (32, ^7,) from the Tiburon Penin- 

 sula, north of San Francisco, where it forms an important 

 constituent of a particular crystalline schist. It has been 

 found also in glaucophane-schists near Berkeley and else- 



