138 BULLETIN 118, UNITED STATES NATIONAL MUSEUM. 



with the albitic feldspar and the lepidolite, as in the examples from 

 Portland, Connecticut. 



Other associated minerals shown are lithiophilite from Grafton, 

 New Hampshire (88253); apatite from Strafford, New Hampshire 

 (87435); allanite from Crown Point, New York (90262); gadolinite 

 from Barringer Hills, Texas (88442); and uraninite from Mitchell 

 County, North Carolina (59329). 



B. THE MESA GRANDE, CALIFORNIA, PEGMATITES. 



The California series was prepared with great care and elaboration 

 by Dr. W. T. Schaller, of the United States Geological Survey and at 

 the same time honorary custodian of gems and precious stones in 

 the National Museum. The details of the collection as given by him 

 are as follows: 



The nearly flat-lying pegmatite dikes, from which most of the specimens shown were 

 obtained, crop out on the hills north and east of Pala, San Diego County, California, 

 and are of the compound, unsymmetrical type whose different parts are thought to be 

 due to differentiation processes rather than to multiple injections of material into 

 reopened fissures. The upper portion of the dike is locally known as the "top rock " 

 (nos. 89549, 89550) and is a mixture of a coarse, granular aggregate of quartz and 

 feldspar and of a graphic pegmatite. No gem stones are found in this "top rock." 

 The lower portion of the dikes, locally called the "bottom rock " (no. 88551), is a much 

 finer grained granular quartz-albite rock characterized by numerous wavy bands of 

 brownish-red garnets. These bands lie nearly horizontal in their general trend, 

 being parallel to the slight dip of the dike. The "bottom rock" is likewise free from 

 gem stones, but both it and the "top rock" are of great interest from the scientific 

 point of view. Between the "top rock" and the "bottom rock" is the middle portion, 

 called the "pay streak" by the miners, in which the gem minerals of value are found. 

 Here also occur the cavities or pockets which often yield an abundance of the well- 

 crystallized minerals shown in the exhibit. 



The top horizontal shelf on the south side of the exhibition case contains the 

 granitic (no. 89871) and gabbro country rock (no. 88556) and also the partly altered 

 gabbro — a loose, friable rock— and the completely altered gabbro (no. 89856), which as 

 a brown iron-stained clay has been washed into the cracks and seams of the pegma- 

 tite rock. Where such a crack extends into a pocket the clay has coated the gems and 

 associated minerals found therein. There are shown, for example, white feldspars of 

 the pegmatite coated with the brown clay derived from the gabbro country rock. 

 The origin of the clay of the gem pockets is thus explained. 



The second horizontal shelf on the south side contains different varieties of the "top 

 rock," consisting of graphic pegmatite and granular pegmatite. The third horizontal 

 shelf shows the mineral aggregates of the middle part or "pay streak," which yields 

 on decomposition the loose, friable material forming the gem pockets. This same 

 shelf also shows examples of the banded "bottom rock." 



The sloping shelf on the south side contains large specimens of the different varieties 

 of the pegmatite rock, including granular, graphic, and banded pegmatites. Several 

 of these larger specimens have been sawed and polished and are well adapted for use 

 as an ornamental stone, especially when cut obliquely so as to form wavy lines and 

 circular effects resembling bird's-eye wood. One specimen in particular consists of a 

 large section of the entire pegmatite dike and shows the aggregate of lithium minerals 

 in the upper portion or "top rock," the granular pegmatite of the middle portion, and 

 the banded "bottom rock." (No. 88560.) 



