58 W. S. Bayley — Rooks of Pigeon Point, Minnesota. 



The third variety was noticed more particularly at the con- 

 tact with an olivine gabbro, which occurs on the point in 

 larger masses. As the red rock approaches the gabbro it i& 

 clearly seen to be affected by the latter in such a way as might 

 be expected if both rocks were in a pasty condition at the same 

 time, or if one had been intruded in or next to 



the other under enormous pressure. The red 

 rock becomes darker as it approaches the gabbro. 

 The green spots, which are scattered over the 

 red groundmass, become more prominent. They 

 are larger in size and more abundant in number 

 than in the two varieties above described, and 

 in some cases are united into red-like bodies and 

 arborescent forms (fig. 2). A light colored feld- 

 spar is also much more frequently discernible in this variety. 

 Still closer to the gabbro a rock is observed which is very 

 dark in color, and can be distinguished from the gabbro only 

 by the possession of a reddish feldspar among its components. 

 The darkest of these rocks resembles very closely the orthoclase 

 gabbros* of Irving, which are supposed by Dr. Wadsworthf to 

 be but altered forms of olivine-gabbro. A discussion of this 

 point can not be entered upon in this place, but it is hoped 

 soon to obtain results from the work now being carried on, 

 which will determine whether or not the orthoclase gabbros may 

 have been derived by the action of an acid magma upon a basic 

 gabbro with which they are always associated. 



The lighter colored of these intermediate rocks (as we shall 

 call them for the sake of brevity) when examined under the 

 microscope are found to differ but little from the red rocks de- 

 scribed above. They contain a larger amount of plagioclase 

 (oligoclase ?), of chlorite, and of biotite, and much more mag- 

 netite and apatite than do the latter, but otherwise resemble 

 them very closely. Micropegmatite is more frequent than is 

 the granophyre intergrowth of quartz and feldspar, and it is es- 

 pecially to be remarked that in almost every case examined the 

 extinctions of the little quartzes are in the direction of their 

 longer axes. The most noticeable fact in relation to them is the 

 freshness of their plagioclase, which is usually in large tabular 

 or lath- shaped crystals. 



"When examined carefully and compared with sections of Irv- 

 ing's augite-syenites they are seen to bear a strong resemblance 

 to some of these — a resemblance so strong that pictures^ repre- 

 senting the augite-syenites might as well be used to illustrate 

 the appearance of the Pigeon Point rocks under the microscope. 



* Copper-Bearing Rocks, p. 50. 



•f L. c, p. 54. Cf. also Herrick et al., American Geologist, June, 1888, p. 340. 



\ Copper-Bearing rocks, p. 112, and figs. 1, 2, 3, 4, PI. XIV. 



