344 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOIy. 11, NO. 14 



to a less degree in the lath-like forms. Twinning is observed in one 

 or two of the crystals. 



Olivine, originally present in amount roughly estimated at 18 per 

 cent, is completely altered to greenish yellow serpentine, but the 

 outlines of the original mineral are usually well preserved, 



CHEMICAL, COMPOSITION 



The composition of the rock is shown in the chemical analysis 

 given in table 1 . It is regretted that fresher material was not available 

 for analysis, for the high water content (5.21 per cent) indicates 

 considerable alteration of the rock, a fact fully confirmed by microscopic 

 study. 



TABLE 1. — Analysis and Norm of Lamprophyre Dike, Louisa County, Virginia. 



S. D. GoocH, Analyst 



Si02 37.40 or 15.01 



AI2O3 18.60 ab 4.19 



Fe203 6.35 an 40.87 



FeO 7.95 di 0.68 



MgO 3.80 hy 11.67 



CaO 11.57 ol 0.48 



Na20 .' 0.50 mt 9.28 



K2O 2.53 il 7.45 



H2O- 1.89 ap 4.70 



H2O + 3.32 



Ti02 : 3.89 Symbol: II(III).5.4.2. 



P2O5 2.04 



MnO 0.23 No name for II. 5. 4. 2. in the 



- — ■ — "Quantitative System." 



100.07 



Sp. Gr 3.065 



Because of the altered condition of the rock the norm and mode 

 do not agree. There is no name yet for the position in the "Quantita- 

 tive System" of classification in which the rock falls, or for the next 

 one in salfemane, but it is so altered (over 5 per cent of water) that it 

 seems not advisable to propose one. The high K2O is undoubtedly 

 contained in the biotite; Ti02 mostly in the titaniferous magnetite or 

 ilmenite, but partly in the augite, and probably to some extent in the 

 biotite ; while the high P2O5 is derived from apatite, which is present in 

 larger amount than is usual for the average rock of this type. 



AGE 



The exact age of the dike is unknown. The ore body which it 

 cuts is one of several of similar composition occurring in the same 

 district in Cambrian schists. The pyrite bodies are genetically re- 

 lated to intrusions of granitoid rocks that may be later but not older 



