PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 213 



Aporhyolyte.] 



become stained is the same that is minutely crystalline in No. 2. The reddening 

 ingredient is more or less abundant in the same structure throughout. When it 

 becomes scant, the minutely crystalline mineral becomes apparent; when it is 

 wanting, the minutely crystalline substance has its typical appearance. It looks as 

 if these may have both been produced by an alteration of an original glass, the 

 reddened aspect being due to the access of much ferric oxide. 



But the minutely crystalline substance has two forms. One is streamed, and 

 changes to opaque red by increase of ferric oxide. This gives a finely flecked polar- 

 ization when it is clear, but in the main, between crossed nicols it is gray or nearly 

 dark. The other part gives a minutely fibrous polarization, and has a yellowish 

 tint. This second part has a close relationship with the porphyritic crystals, which 

 it sometimes entirely surrounds in a series of undulatory concentric bands, thus 

 excluding part No. 1 entirely from contact with the crystals. This undulatory 

 surrounding is not always a fluidal structure, but a secondary growth. It occurs in 

 many instances in isolated areas, without the presence of the crystal, and it is in all 

 cases abruptly separated from the other minutely crystalline part. In the gross this 

 second part of the minutely crystalline substance produces the evident magascopic 

 fluidal form, but its minute structure is fibrous independently of that, and it has a 

 preference for the proximity of the crystals. It is probably a segregation from the 

 magma, later than the crystals, but not able to assume a crystalline form. Its fibres 

 have a parallel extinction and a positive elongation. 



The feldspar crystals are the most interesting portion. Some years ago the 

 writer made an examination of these crystals and came to the conclusion that they 

 were adiilaria, a form of orthoclase, and so published it. Subsequently, Prof. Irving 

 declared them to be quartz, and without any further examination Irving's deter- 

 mination has been accepted. This rock, and the rock of the Great Palisades which 

 immediately overlies it, really constituting one bluff, has usually passed for quartz- 

 porphyry, on the authority of Prof. Irving. Pebbles from this rock constitute 

 great conglomerates, being very durable, particularly on the south side of lake 

 Superior. 



A close inspection of these crystals with a common loop shows occasionally, 

 though not usually, a cleavage parallel to one of the sides. The forms are not hex- 

 agonal, but this fact was presumed to be due to corrosion by the magma. Their 

 sections in the slide are not hexagonal. They are of all shapes, such as a monoclinic 

 crystal would give. They are never twinned, but simple, glassy, resembling quartz in 

 having a conchoidal fracture and in the colors of polarization. 



In one of the slides examined is a section perpendicular to n g , quite character- 

 istically manifested. It has an extinction angle with cleavage of 9. 



