82 



University of California Publications. [Geology 



LAWSONITE. 



Introduction. — New minerals are frequently discovered and 

 the list of mineral names has largely increased since the appear- 

 ance of Dana 's System of Mineralogy in 1892. With very few 

 exceptions, however, the minerals discovered since that date are 

 to be classed as rare because they seldom occur abundantly and 

 are usually confined to a single locality. 



The discovery of a new mineral which is essentially a rock 

 constituent is still rarer. While the introduction of new rock 

 names is very prevalent, it is well known that such names are not 

 generally based upon the presence of new constituents, but upon 

 the presence and relative proportions of the old long known rock- 

 forming minerals. 



In the case of lawsonite we have a mineral of recent discovery 

 of fairly wide distribution as a rock-forming mineral, and it is a 

 matter of some surprise that it so long escaped the observation of 

 petrographers. 



Lawsonite was discovered in 1894 by Ransome and Palache 1 

 at Reed 's Station, Marin County, California, as a deposit of large 

 well developed crystals, and as a constituent of the glaucophane 

 schists in that locality. This occurrence of large crystals is the 

 only one so far known, but as a rock constituent it is interesting 

 to note that almost coincident with its discovery by Ransome, the 

 mineral was observed by Pranchi 2 in the glaucophane rocks of 

 the Piedmont district, Italy, and by Lacroix 3 in the saussuritic 

 gabbros from Corsica, both writers noticing its presence as an un- 

 identified constituent of their rocks. 



Franchi observed the mineral as an unknown constituent asso- 

 ciated with albite and muscovite, resulting from the alteration of 

 the feldspars of an altered ophitic diabase found in the Maira 

 Valley of the Piedmont Alps. He later mentions the same un- 

 known minerals as having been found by Stella in little veins in a 

 soda amphibolite, from the high valley of Chiana. 4 This uniden- 



1 Bull. Dept. Geol. Univ. Calif., 1S95, 1, 301-312. Zeits. fur Kryst., 1896, 

 25, 351-357. 



2 Bollet. del K. comit. geol., 1895, 2, 90. 



3 Miner, de la France, 1895, 1, 708. 



1 Bollet. della Soe. geol. Italiana, 1896, 2. 



