266 



University of California. 



[Vol. 3. 



Mode of Occurrence. — The formation in which these fine 

 crystals are found at the Pala locality consists of a pegmatite dike, 

 dipping westerly at a low angle, perhaps 20? It is more or less 

 broken, and, as a whole, seems to form the surface of much of 

 the slope of the hill, on which it occurs. The dike is rather 

 broad, but irregular in its present shape, and has a thickness of 

 probably not more than thirty feet. Plate 25 is a photograph of 

 the locality, looking north-west. 



So far as the mining developments have shown, only a small 

 portion of the dike is rich in lithia minerals. Ordinarily, the 

 dike is a coarse muscovite granite, the orthoclase and quartz 

 predominating, containing many rounded prisms of black tourma- 

 line, with broken ends. Lepidolite occasionally seems to replace 

 the muscovite and when it does, red, blue and green tourmalines 

 replace the black variety. It is with these gem tourmalines that 

 the spodumene occurs. While the tourmaline and lepidolite are 

 frequently inclosed in the quartz and feldspar, no such inclusions 

 of spodumene have been found. The latter mineral always 

 occurs associated with the other minerals, but never penetrating 

 them or penetrated by them. It occurs in pockets and these 

 facts seem to indicate that the formation of the spodumene is 

 later and not coincident, in time of formation, with the tourma- 

 lines and with the dike. 



The dike cuts across the large intrusion of dark rock occuring 

 at Pala and briefly mentioned by Dr. H. W. Fairbanks* This 

 large body of dark rock, several miles across is surrounded on 

 all sides by granite. It is in this same body of rock and hardly 

 a mile from the spodumene locality that the well known 

 lepidolite mine is situated from which so many specimens of the 

 fine-grained white lepidolite with the radiating groups of rubellite 

 have been obtained. This large body of lepidolite occurs in a 

 similar pegmatite, having the same general strike and dip as the 

 dike carrying the spodumene, which mineral has not as yet 

 been found in the lepidolite mine. 



The dark rock forming the footwall of the dike in which the 

 spodumene occurs, is a diorite, consisting of hornblende, a plagio - 

 clase, and (subordinate) orthoclase with accessory magnetite 

 and apatite. 



* Ninth Report State Mineralogist (California.) 



