PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 493 



Diabase. Gabbro.) 



"Is a gray crystalline rock with its feldspar turned chiefly to a gray or pink 

 color. Under the microscope the feldspar is seen to be in lath-shaped crystals, with 

 triangular interspaces containing magnetite, cir/dite, etc. The feldspar is much altered 

 and filled with kaolin and micaceous scales; but in places it is clear, showing the 

 polysynthetic twinning of plagioclase. Some secondary quartz and feldspar occur, 

 and the pyi'oxenic constituent is replaced by viridite." 



One section examined. 



Aye. Cabotian. u. s . o. 



No. 690. DIABASE. 



Near a small lake at the north end of the portage from Caribou lake. This small lake is called Little lake 

 (or Straight lake) and lies in the N. W. ^ sec. 18 and S. W. % sec. 7, T. G4-1 W. 

 Ref. Annual Report, x, page 79. 



Meg. A fine-grained, very dark-gray, compact diabase. 



Mic. The section is composed of lath-shaped plagiodases, uiu/itc in ophitic grains, 

 inaj/iietite and confused grayish, greenish and yellowish alteration products. Some 

 of the secondary materials clearly were derived from the augite, and it seems most 

 probable that the rock was originally a holocrystalline aggregate of the three minerals 

 mentioned above. 



One section examined. 



Age. Cabotian. u_ S- G 



No. 691. GABBRO (with olivine). 



Same locality as No. 090. "This is a sample of the usual 'trap' of the country. It weathers light, and is 

 in that way in marked contrast with the last, though that difference seems to be due solely to the greater pro- 

 portionate amount of feldspar in this, and of augite in that. This occurs in patches on and in the last, and 

 especially on the north side. It also seems to cross and cut it in vanishing veins." 



Ref. Annual Report, x, page 79; Bulletin ii, pages 92, 93, plate XII, figure 2. 



Meg. Hand specimen not found. 



Mic. M. E. Wadsworth's description of this slide is as follows:* 

 "The section is composed of feldspar, diallage, magnetite and .olivine, and is 

 similar to No. 692. The olivine is largely altered to a yellowish-green fibrous 

 serpentine. This alteration has proceeded in a peculiar manner. In some of the 

 olivine grains are bands of black and brown plates and needles, which for the most part 

 are arranged parallel to the axis of the greatest elasticity, although a few are at 

 right angles to it. The olivine also has two well-marked cleavages parallel to these 

 directions, along which lines the serpentinous alteration extends. Cut as the alter- 

 ation is the greatest at right angles to the line of greatest elasticity, the more highly 

 altered olivines present an alternate series of parallel bands of wr/ia/fine and partially 

 altered olivine. In some places the planes of greatest alteration coincide with the 

 line of greatest elasticity, while in some of the entirely changed olivines the serpen- 



*Bullftin ii, pp. 92, '., ]>liiti> XII, lifriin- 2. 



