466 THE MARQUETTE lEON-BEAKOG DISTEICT. 



original character. It is believed, however, that they were basic porphyrites 

 which occurred either in volcanic necks or as moderately thick lava flows. 



The coarse-grained greenstones are the fillings of volcanic orifices, 

 or they constitute great dikes cutting through the bedded members of 

 the Clarksburg series. In these the green amphibole is sometimes in 

 large, well-characterized, ophitic areas, between which are the alteration 

 products of plagioclase. The interior of the hornblende, which is more or 

 less chloritized, is of a yellowish-green color, and ai'ound tliis is a periphery 

 of dark-green amphibole, more particularly where the mineral is in contact 

 with undoubted remnants of plagioclase. The color of both micleal and 

 peripheral amphibole is bluish-green in a direction approximately parallel 

 to the cleavage, but the peripheral hombleude is darker than the nucleus. 

 Around the borders the plates are all fringed with long needles of the gi*een- 

 blue amphibole, and similar needles penetrate in all directions the materials 

 of the light interstitial substance between the amphibole plates. This inter- 

 stitial mass is an aggregate of the decomposition products of plagioclase, 

 among the more prominent of which are biotite, calcite, epidote, and quartz. 

 In addition to the long spicules of gi-een-blue amphibole that cut this 

 aggregate, others with the characteristics of actinolite also penetrate it. 



In a very few cases a typical diabasic structure is noticed on weathered 

 surfaces of ledges. In the thin section of these rocks, however, a porphy- 

 ritic structure is also observable. Decomposed feldspars with quadratic 

 cross-sections are embedded in the usual plexus of hornblende, biotite, 

 altered plagioclase, quartz, and magnetite, to which is often added kaolin. 

 The plexus may be in areas with an ophitic outline, but the lines between 

 the porpliATitic crystals and the matrix in which they lie are rendered so 

 obscm-e by the many secondary substances that have arisen from the alter- 

 ation of the feldspars that it is difficult, and in many cases impossible, to 

 make them out. The larger crystals of plagioclase, of whose feldspathic 

 nature there can be no doubt, are so filled with kaolin, sericite, quartz, 

 biotite, and hornblende that it would seem probable that many of the same 

 minerals in the matrix must have likewise been deiived from feldspar. 



Certain compact hornblendic rocks diff"er from the specimens just 

 described simply in the possession of a great quantity of amphibole. The 



