518 PROCEEDINGS OF THE AMERICAN ACADEMY. 



a multitude of small parallel crystals which give the surface a pitted 

 appearance. 



A thin section cut across this boundary showed under the microscope 

 that the outer opaque layer was in crystallographic continuity with the 

 inner part, but the line between them was sharp, the opaque layer con- 

 taining very abundant liquid and obscure solid inclusions and showing 

 faintly a division into libres or columns perpendicular to the surface. 

 The appearance pointed to a second period of quartz deposition rather 

 than to an alteration of quartz previously formed. The evidence of 

 the secondary deposition of the white quartz was rendered stronger by 

 the occurrence in several places between the two layers of a thin de- 

 posit, not more than 1 mm. thick, of tiny muscovite crystals, or of a 

 double layer of muscovite and cookeite. The apatite crystals are often 

 deeply embedded in the white quartz and seem to have developed in 

 part pari passu with this material ; but at no place does the apatite 

 appear to penetrate the smoky quartz. 



The lepidolite is in part in confused lamellar aggregates, in part in 

 quite definite hexagonal prismatic crystals with somewhat rounded basal 

 terminations. The interior of these crystals is of the characteristic 

 pale lilac color of lepidolite, but their surfaces are everywhere covered 

 by a uniform layer of pale greenish-white muscovite about 1 mm. in 

 thickness. The cleavage of the lepidolite and of the bordering musco- 

 vite is absolutely continuous, but the boundary between them is sharp 

 and plane, showing that the muscovite represents, not an alteration of 

 lepidolite, but a later parallel growth of the new and isomorphic mineral, 

 a sort of secondary enlargement. Sharply bounded lepidolite crystals 

 without the muscovite border are sometimes enclosed in the smoky 

 quartz, showing that these two minerals were of contemporaneous 

 growth. 



The orthoclase, a pale flesh-colored variety, and the albite, colorless, 

 in thin plates showing albite twinning, are small in amount, and their 

 relations to the smoky quartz show that they belonged to the same 

 period of growth with it. 



Cookeite occurs quite abundantly on some of the specimens as crusts 

 or clumps of scales or platy crystals of a greenish-white color. It is 

 similar in appearance to the muscovite, but is slightly darker in color, 

 less pearly in lustre, and readily distinguished by its reactions before the 

 blowpipe. The cookeite appears to have been formed at several periods 

 of mineral growth in the cavity. It is seen occasionally as above stated 

 in thin layers between the outer white quartz layer a»d the coating of 



