2i8 TJie Auicrican Geologist. October, looo 
beautifully varying in regular bands, perpendicular to the 
fibres, from porcelain white to bright pink, and less commonly 
to clear green either pale or dark. These amygdaloidal ze- 
olites, while composed of minerals of little hardness, are usual- 
ly so interlaced as to form the most durable part of the rock, 
and many may be gathered on the beach of lake Superior, all 
derived from the disintegration of plagioclasytes and similar 
gabbro rocks. The finest of these pebbles have found a ready 
sale among the jewelers of the region, who use them for va- 
rious decorative purposes. 
The TEXTURE of the plagioclasyte is always granitoid and 
holocrystalline. It varies from very coarse to medium grain, 
but is generally coarse. Cleavage faces of the constituent 
feldspar frequently measure one to two centimeters and may 
even attain four or five. The feldspar is never porphyritic nor 
ophitic, though occasionally the accompanying rare augite 
shows a tendency to automorphic forms. 
In thin sections the coarseness of the rock is at once appa- 
rent for it is not rare that one or two grains of feldspar occu- 
py the whole of a section. At the same time its great predom- 
inance is evident, as the augite only occurs as rather small 
crystals surrounded by, and of earlier growth than, the feld- 
spar, except in the rarer cases when it occurs here and there 
in small rudely triangular areas, or in narrow lines between 
the large plagioclase crystals. Indeed it may be stated in gen- 
eral that in Minnesota plagioclasytes a triclinic feldspar of the 
labradorite series makes up from three-fourths to nine-tenths , 
of the rock. Samples can be found exclusively composed of 
feldspar, though it is rare that more or less magnetite does- not 
occur. However, the normal plagioclasyte contains also more 
or less augite, and thus passes by insensible gradations into 
an ordinary normal gabbro. Apatite also is rarely absent, 
though it often occurs as only one or two small crystals. Oliv- 
ine very rarely occurs in ordinary plagioclasyte. Irving 
claims to have seen "a few grains." Thus the only primary, 
minerals observed are: 
Apatite. 
Magnetite. 
Augite. 
Labradorite. 
