290 PROCEEDINGS OF THE AMERICAN ACADEMY 



veins which traverse both the matrix and the chlorite of the amj'g- 

 dules. In doing this, it penetrates between some of the Limince, and 

 encloses others in a manner that proves it to be younger than the 

 chlorite. 



Bed No. 64. — The amygdaloid of bed 64 of the Eagle River Sec- 

 tion has about sixty per cent of its volume occupied by amygdules, 

 sometimes wholly prehnite, sometimes an outer layer of white prehnite, 

 and a central filling of calcite. The matrix is chocolate-brown, and 

 has a crystalline texture wholly foreign to the melaphyres, and more 

 resembling that of a fine-grained, somewhat oxidized spathic iron ore. 

 Its hardness is 6 ; fusibility 2-2.5 ; it dissolves in muriatic acid, leaving 

 pulverulent silica, and the solution contains abundance of alumina and 

 lime ; in thin sections it is seen to be clearly orthorhombic, and polar- 

 izes the light with the same colors as prehnite, which it undoubtedly 

 is. In thin sections, by ordinary light, the first things we see are the 

 characteristic outlines of plagioclase crystals, filled with a limpid color- 

 less substance, while all the interstitial spaces are filled with a less 

 clear substance, colored brown by countless particles of iron oxide. 



Examining it between crossed nicols, a remarkable change takes 

 place. The plagioclase outlines are still sharply defined by the abun- 

 dance of particles of iron-oxide suspended in the interstitial substance ; 

 but every thing except these brown particles is changed to prehnite. 

 In places, the feldspars are each occupied by a fine-grained aggregate 

 of prehnite ; but often the latter mineral has crystallized more freely a 

 group of long, radiating, tabular individuals, reaching with brilliant red 

 and green colors across whole groups of plagioclase crystals and the 

 interstitial spaces, and sometimes well into an amygdule without a 

 break in the integral polarization of each plate. 



Below the amygdaloid of bed 64, just described, there are several 

 beds (with an aggregate thickness of twenty feet) forming apparently 

 a transition into a pseudo-amygdaloid. The rock of these beds has a 

 more or less light green color, a compact, aphauitic matrix, in which 

 lie abundant amygdules \ inch and less in size. Many of these are 

 filled with prehnite; as many more are cavities lined with rosy crys- 

 tals of adularia, while others contain both of these minerals. The feld- 

 spar crystals in the cavities are sharply defined prisms, terminated at 

 the free end with the basal plane. In thin sections, the matrix contains 

 much pyroxene unaltered, except that it is much broken. 



The plagioclase is all much altered, and a considerable proportion of 

 the crystals is changed to chlorite. Where they are still colorless, they 

 have lost the twin striation in polarized light. 



