246 W. S. BAYLEY SYENITES FROM NEW ENGLAND. 



first and Inst mentioned minerals are no doubt alteration products of the 

 augite. The bright green hornblende is strongly pleochroic in bright. 

 green tints in sections parallel to the vertical axis and in green and 

 brownish-green tints in basal sections. The cross-cleavage of hornblende 

 is very apparent in the latter, and sometimes this is accompanied by the 

 rectangular cleavage of augite. The inclusions, of this hornblende, as of 

 the augite from which it is derived, are apatite and small grains of mag- 

 netite. 



Intermingled with the green hornblende and including large masses of 

 it are large and small plates of biotite, whose strong pleochroism is in 

 very dark brown and bright yellow colors. Its extinction, determined . 

 by means of the quartz ocular, is parallel to the cleavage, but its axial 

 figure opens slightly when revolved between crossed nicols. There is no 

 evidence that the mineral is an alteration product of augite. Its rela 4 

 to the green hornblende and leucoxene which it inclosed declares it to . 

 younger than these, or, more properly speaking, than the augite froi 

 which these are derived. In addition to the green hornblende and the 

 leucoxene* the biotite also includes crystals of apatite and sphene that 

 are probably original separations from the magma. 



Another form of the biotite is surrounded by green hornblende in such 

 a way that we must suppose a small quantity of the latter to have resulted 

 from the alteration of the former, for the borders of the mica, like those 

 of the augite, are fringed with a narrow rim of the hornblende. 



Of the nature of the brownish-green hornblende but little has been 

 learned. It is frequently in idiomorphic grains, bounded by the usual 

 forms found on hornblende, and is often twinned according to the ordinary 

 law. Its color in prismatic sections is dark green, with a slight tinge of 

 yellow in a direction highly inclined to the cleavage, and dark brown, 

 ah nost < >] >aque in directions nearly parallel to it. In basal sections the ray 

 parallel to a is dark green, while that parallel to I) is almost completely 

 absorbed. The scheme for the absorption is consequently C = fc>a. 

 The extinction is high, certainly above 24°, and the inclusions imbedded 

 in the mineral are those common to the other bisilicates. Around its 

 edges are sometimes discoverable little masses of iron oxides that may 

 indicate magmatic resorption. This variety of hornblende was seen in its 

 greatest perfection in the slide made from Hawes' original specimen. 

 Here it occurs not only in the aggregated basic concretions, but also in 

 isolated idiomorphic grains, commonly associated with eleolite or its de- 

 composition products. It is also abundant in the specimens obtained by 

 Mr. Smith. From the fact that the mineral occurs so frequently in isolated 



*The distinction here made between tin 1 two titanium minerals is merely one of origin, tin' 

 granular secondary substance being called leucoxene, and tin- < rystallized original litanate being 

 dci inated sphene. 



