502 PROCEEDINGS OF THE NEW YORK MEETING 



erals — 20 or more sjjecies being occasionally observed in a single dike — and in 

 other regions few of tlieni are to be found. Thus in the dikes of Mitchell and 

 Yancey counties, North Carolina, these accessory minerals are both numerous and 

 varied, 47 or more being enumerated by Pratt, of which the following most com- 

 mon 20 species may be named, approximately in the order of their abundance : 

 kaolin, almandite, and andradite (garnets), tourmaline, beryl, apatite, epidote, 

 biotite, microcline, orthoclase, allanite (?), uraninite, gummite, autunite, colum- 

 bite, samarskite, zosite (var. thulite), pyrite, magnetite, menaccanite. But in 

 Macon county, over 100 miles southwest, accessorj^ minerals are exceedingly rare. 



In the Black Hills region of South Dakota these mica dikes contain a number of 

 accessory minerals, but not in such variety as has been found in the North Caro- 

 lina region. The following species were observed there by the writer: tourmaline 

 (mostly black), microcline, albite— oligoclase (sparingly well crystallized), quartz 

 (massive — pink or rose), muscovite, lepidolite, biotite, epidote, garnet (rare), colum- 

 bite, wolframite, spodumene (large rough crystals), apatite (rare), menaccanite, 

 cassiterite. Two additional species, triphylite and leucopyrite, have been reported 

 by Blake. In the Cribbensville district. New Mexico, there are but few accessory 

 minerals and these are rare. 



The country rock immediately adjacent to these pegmatite dikes have as a rule 

 undergone but slight changes. Frequently they are somewhat impregnated with 

 the quartz and feldspar of the dike, these materials being sufficiently abundant in 

 some cases to give the walls of the dike for several inches from the contact a 

 grayish or whitish appearance, although the country rock may have been a darker 

 color. The alteration as a rule does not extend out more than one or two feet 

 from the dike, generally not more than a few inches. After this jierhaps the most 

 common feature is what may be called the tourmalinization of the country rock 

 to a distance of from a few inches to one or more feet distant from the rock, the 

 tourmaline in small grains or crystals being at times quite abundant next the dike 

 and gradually diminishing as this distance increases. As a rule, the crystals 

 (" blocks" or " books ") of commercial mica need not be looked for in the dikes 

 or veins under two feet in diameter, but there are cases in which crystals two by 

 two and one-half feet in diameter have been found in the dikes, the width of 

 which was scarcely greater than these figures. On the other hand, in case of some 

 of the larger dikes no mica whatever of commercial value has been found, as, for 

 example, the large dike being mined for kaolin by the Harris Clay company, near 

 Webster, North Carolina. 



As to the distribution of mica in the dike, it may be said that generally these 

 crystals are scattered irregularly through the matrix of quartz and feldspar, and 

 in mining operations a large amount of useless material has to be blasted down 

 and I'emoved from the mine in order to secure the commercial product. In other 

 cases, however, the crystals of mica occur in the outer part of the dike near the 

 wall rock, and in such cases the mica " lead " may be more easily followed. In 

 some cases the mica constitutes as high as 10 per cent of the total mass of the 

 dike, but in the majority of cases it will prove to be less than one per cent of the 

 total, and in many of the largest pegmatite dikes, as stated above, no commercial 

 mica is found, the mica present being in small scales and crystals, the largest 

 diameter of which would i-arely exceed two inches. Of the mica taken from the 

 dike in ordinary mining operations, usually less than 10 per cent, and sometimes 



