OP ARTS AND SCIENCES. 283 



have sharply defined, evenly curved contours, differing wholly in this 

 respect from the pseudo-amygdules of the lower zones of the same bed. 

 Often also the primary minerals of the matrix are much more minute 

 in the immediate neighborhood of the amygdules than away from them, 

 and their disposition seems also to have been influenced by the presence 

 of tlie amygdaloidal cavity. The color of the amygdaloid matrix 

 varies between different shades of dark greenish brown, chocolate- 

 brown, dull light green, and brilliant light green. All these colors often 

 occur in the same bed, or even in the same specimen, and, like the 

 differences in hardness, — from 3 to 7, — are due to metasomatic 

 changes. 



There is one variety — the scormceous amygdaloid — which occurs 

 less often than the others, but is always strongly characterized. It 

 consists of true amygdaloid and sandstone curiously associated. The 

 amygdaloid is in contorted forms, from an inch or less to several feet 

 in size, with a sharply defined, often wrinkled, surface, and wholly en- 

 veloped with sandstone. The glaciated surface of the outcrops of this 

 rock often present at the first glance the appearance of a conglomerate, 

 in which here the amygdaloid, there the sandstone, appears to form 

 the cement. Thin sections show that we have to do with a true sand- 

 stone of quartz and feldspar derived from quartz porphyry, and identical 

 with that which forms the great sandstone beds of the copper series ; 

 and that the other constituent is a true amygdaloid. 



The conditions, as studied on the ground, indicate that these beds are 

 volcanic scoria?, buried in the littoral sand. To this variety belong the 

 famous "Ash-bed" of the Copper-falls and Phqeuix mines, and the beds 

 worked in the Hancock and South Pewabic mines. 



Bed JVo. 87 of the Eagle River Section has an actual thickness 

 of 154 feet, of which 7 feet belong to the amygdaloid, 23 to the pseudo- 

 amygdaloid, and 124 to the lower zone. The lower zone has a rather 

 fine grain, and shows to the naked eye only green crystals of feldspar, 

 and irregular-sliaped small spots of dark green chlorite. These con- 

 stituents give to the rock a dirty gray green color, spotted with dark 

 green. The powder yields some magnetite. 



In thin sections, from the lower zone, we can distinguish plagioclase, 

 augite, an impellucid, dirty gray, unindividualized substance, containing 

 often radiating colorless needles of a feebly double-refracting substance, 

 perhaps apatite ; besides these, there are the abundant secondary 

 products. 



The augite, which is the least altered constituent, is generally highly 

 fissured, and in places altered to its characteristic chloritic product, 



