240 



BULLETIN OF THE 



As seen under the microscope, tPie rock is composed of large and small 



feldspar crystals, magnetite, epidote, calcitc, and a mass of chlorite, 

 viridite, and opacite. The large porphyritic feldspars are twinned plagio- 

 clase, and occasionally Carlsbad twins of sanidin. The minute feldspars 

 of the groundmass flow around them, encroach upon, and sometimes 

 break them. Earely, the groundmass, holding small feldspars, has 

 pushed into a crystal, a little distance on either side, and a tongue of 

 the (original) base, alone, without the small feldspars, passes through the 

 crystal and connects the two intrusions, ~ this connecting tongue now 

 altered, however, to calcite, chlorite, and epidote. The small feldspars, 

 when sufficiently fresh, show the triclinic twinning; but some Carlsbad 

 twins of sanidin and unstriated crystals occm; the former of which 

 cannot be referred to the plagioclaso that, owing to the alteration, docs 

 not show its multiple twinning. 



The degree of decomposition that the feldspars have undergone varies 

 in the different sections : in the freshest rock they contain immense 

 quantities of minute fluid inclusions, characterized by moving bubbles, 

 and occasional larger ones, rounded or irregular in shape, together with 

 inclusions of the base. The latter are cylindrical, or irregularly reticu- 

 lated in form, often zonally arranged in the interior or exterior parts of 

 the crystal ; they arc absolutely identical in shape, and in their relations 

 to the enclosing crystal, with the inclusions of glass or base of the fresh 

 basalts ; they are now altered to magnetite, viridite, and other products. 

 In the smaller feldspars these intrusions generally run through the 

 centre of the crystal, parallel with the twinning-plane. Even in the 

 freshest specimens, the substance of the feldspars is filled with minute 

 microliths, and scales either colorless or of a light greenish color, wdth 

 occasionally some epidote, calcite, or quartz, — generally products of the 

 decomposition of the feldspathic substance proper. In the more decom- 

 posed specimens these products multiply, so that the crystals become a 

 mass of these viriditic scales and fibres (often polarizing with the bril- 

 liancy of talc, or in red and yellow colors), or even of opaque kaolin, 

 while calcite, epidote, quartz, and colorless needles with cross fracture, 

 in part apatite, appear to a greater or less extent The epidote occurs 

 generally in the large feldspars in grains : some of it may originate 

 from the alteration of included minerals ; but of this there is no proof. 

 Occasionally two feldspars interpenetrate each other. 



The only other original mineral, unless it be part of the magnetite and 

 apatite, is olivine. This was found in well-marked, large, and undecom- 

 posed crystals only near the contact of the amygdaloid with the con- 



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