646 FRANK F. GROUT 



enough to show twinning bands. In these the extinction is wavy and 

 angles are low. Both oligoclase and orthoclase are probable. 



Magnetite is seen in all sections and apatite in only one. 



Alteration in most cases has proceeded even farther than in the 

 mottled rocks, and resulted in a pseudo-amygdaloid — a botchy mass 

 of secondary minerals. The original texture is revealed by some 

 pseudomorphs, one of the most peculiar being a nearly opaque 

 yellowish-green earth with the form of plagioclase. However, the 

 pseudo-amygdules are quite abundant, developing from any spot as 

 a nucleus, and expanding as they replace the surroundings with a new 

 layer. Chlorite is the commonest material, varying from colorless 

 to dark blue-green, or yellow, or brown, or gray, and usually spheru- 

 litic. In similar position in other rocks are calcite, quartz, epidote, 

 actinolite, and a dusty red mineral of .very feeble birefringence, which 

 may be laumontite or some other zeolite. Regarding the development 

 of most of these, reference must be had to Pumpelly's paper, but some 

 new work is here offered regarding the laumontite rocks. There is 

 an extensive development of this mineral in some amygdaloidal and 

 vein cavities on Snake River in Minnesota. Its properties are given 

 below with those of other amygdules. Beginning at Kettle River 

 and extending twenty-five miles northeast, is a series of outcrops 

 of a red pseudo-amygdaloid, associated with which are real 

 amygdaloids with red laumontite. The red material of the 

 pseudo-amygdules appears to be similar to that of the amygdules, 

 but the rocks are badly softened and microscopic examination is not 

 very satisfactory. A specific gravity separation of the rock-constit- 

 uents yielded rather mixed material, the lightest fraction being 

 strongest red in color. Chemical tests of the two lightest fractions 

 show that there is an approach to laumontite (Analyses 4 and 5, 

 Table VI), and there is no doubt that this mineral has developed 

 in the rock-matrix as well as in the cavities. Pumpelly suggests the 

 possibility, and thinks it an early product in the alteration. Its 

 associates here are badly altered; only augite, of the original, is still 

 recognizable in the mass, which affords clear evidence of its original 

 texture. 



An entirely distinct line of chemical work has given a hint of the 

 history of some of the rocks. This is the composition of the product 



