280 PROCEEDINGS OF THE AMERICAN ACADEMY 



tion. They often contain many long and straight hair-like black rods, 

 or prisms, ranged strictly parallel to the longer axis of the crystal. 

 Besides these, they contain abundant fluid cavities with dancing bubbles, 

 which did not diminish in size when heated to 95*^ C* 



They enclose also globular or egg-shaped drops of a greenish-brown 

 substance ; on examining these in crystals cut perpendicularly to the 

 longer axis, they were found to be isotrope. There can be no doubt, I 

 think, that these are portions of the original fluid magma, out of which 

 the rock crystallized. We owe to the almost indestructible character 

 of the apatite, and to its lack of double-refraction in one direction, the 

 preservation of this glass, and our ability to recognize its amorphous 

 nature, and thus to obtain the strongest circumstantial evidence of the 

 eruptive origin of these rocks. 



The relative ages of the constituents may be represented by the 

 following scheme: — 



(I.) 1. Apatite. 2. Oligoclase and ORrnocLASE. 3. Augite and Magnetite? 



\ ■ \ . 



(n.) c. Fekbite. J J Chlorite e. Quartz. (Yellowish- 6. Magnetite? 



■ i:«:~o „ ) green 



mineral. 



( monoclinic? a. < green 



Bed No. 61. — Among the changes that have resulted in the forma- 

 tion of pseudo-amygdaloids is one which seems to be rare, for I have 

 met with it in only one of the several hundred sections I have cut of 

 these rocks. It is the formation of pseudo-amygdules of analcite on a 

 large scale. The rock is from the loioer part of bed No. Gl of the 

 Eagle River Section. It is greenish-gray, and breaks with an uneven 

 and somewhat schistose fracture. It is filled with irregularly shaped 

 grains of a white mineral, which fuses, at about three, to a clear glass 

 containing air bubbles ; it dissolves in muriatic acid, leaving a deposit 

 of silica, and the filtrate gives with ammonia an abundant precipitate 

 of alumina; in thin sections it is transparent and colorless, and shows 

 its isotrope character by remaining dark during a revolution between 

 crossed nicols. The mineral is undoubtedly analcite. Isolated bands 

 occur, showing very feeble double-refraction. 



In thin sections, besides the analcite, the most conspicuous constitu- 

 ent is the green, soft, chloritic substance characteristic of the pseudo- 

 amygdules. In this lie isolated plagioclase crystals, some showing 

 twin striation in polarized light, and others in which this has been 

 lost. Optical measurements on these crystals, on sections cut in the 



* Tliis is not corrected for the influence of Beck's Jj immersion objective. 



