14 DESCRIPTIVE GEOLOGY. 



accessory minerals there occur sparingly in the gabbro minute plates of 

 yellowish- white mica, usually associated with the diallage. Ilmenite and 

 magnetic iron are present in rounded grains; these are frequently decom- 

 posed on the surface, and give the rock a somewhat mottled appearance. 



Under the microscope, the labradorite crystals show in a striking man- 

 ner many of the minute structural peculiarities observed in the labradorites 

 of the Scandinavian and German gabbros. For a detailed comparison of 

 these resemblances, see Professor Zirkel's report, page 108. In its min- 

 eralogical and chemical features, as well as in its geological occurrence, this 

 gabbro bears a very close analogy to rocks described by Scheerer^ from 

 Anabel and Hitteroe Islands and other localities in Norway. 



Eocks with nearly the same habit have been mentioned as occurring 

 in the Laurentian series of Canada by the geological survey of that country. 

 A chemical analysis of the Iron Mountain gabbro, made by Dr. Wieder- 

 mann, in Leipsic, seems to indicate clearly that but little diallage can be 

 present. It has nearly the composition of pure labradorite. 



Below are given two analyses, for the purpose of comparison (No. 1, 

 the Laramie Hills gabbro; No. 2, a rock described by Dr. T. Sterry Hunt^ 

 under the name of labradorite, from Chateau Richer, in Canada) : 



No. 1. No. 2, 



Sihca 52.14 55.80 



Alumina 29.17 26.90 



Ferric oxide 3.26 1.53 



Lime 10.81 9.01 



Magnesia 0.76 0.27 



Soda 3.02 4.77 



Potassa 0.98 0.86 



Loss by ignition 0.58 0.45 



100.72 99.59 



Iron Mountain, to which reference has already been made, is a mass of 

 titaniferous iron, or ilmenite, and is situated just north of the Chugwater 

 Creek, about 1^ miles above the point where the stream leaves the hills. 



' Scheerer, Neues Jahrbuch fur Mineralogie, 1843, 668. 

 2 Geological Survey of Canada, 1863, 478. 



