406 THE GEOLOGY OF MINNESOTA. 



[Mesolite and thomsonite. 



feldspars are much changed by a kaolinic alteration and apparently sometimes this 

 promoted the formation of thomsonite in the immediate vicinity. In such a case 

 the fibres of thomsonite are vertical to the elongation of the feldspars. In the augites 

 are embraced sometimes ten or twenty of such altered microliths. 



The oli vines which are small are also changed largely to bowling ite, which some- 

 times is faintly greenish, but is usually of a dark red or orange red. 



The zeolites constitute the most interesting element in the rock. They are of 

 two kinds which optically are quite distinct, inasmuch as one is almost dark 

 constantly between crossed nicols, and the other is translucent. They are both 

 radiately fibrous, but the former is more fine and more regular in its structure. 

 These are for the most part in separate amygdaloidal masses or areas, but occasion- 

 ally they are seen in the same mass, where they are contrasted in the same 

 manner, though intimately intergrown. The coarsely fibrous zeolite is easily 

 identified as thomsonite. It is elongated both negatively and positively, and extin- 

 guishes parallel to the elongation. The finer zeolite is that which has usually been 

 distributed under the name of thomsonite. It has a parallel extinction which may 

 be observed by a faint increase of its darkness when in agreement with either nicol. 

 Its section, viewed macroscopically, is white, in the slide. It is probably a soda 

 zeolite, i. e., a form of the natrolite group of Dana. These zeolites are discriminated 

 under Nos. 535A and 535B. 



Besides the foregoing, scolescitf forms rounded amygdules in the same rock. 

 Outwardly they resemble thomsonite except that so far as noticed this mineral does 

 not grow into such large masses as thomsonite, but forms small round balls. It 

 is not iron-stained in pinkish bands, like mesolite, but is clear and glassy white. 

 When mesolite is white it is porcellanous. Under the microscope the section of 

 scolescite shows negative elongation and low double refraction, characters that 

 distinguish it from thomsonite. 



Lintonite is green outwardly, very fine and hard, and seems to grow in small 

 rosettes in the mesolite and in larger independent masses. 



Two sections. 



Age. Manitou. N. H. w. 



Nos. 535A AND 535B. MESOLITE AND THOMSONITE. 



There were made nine thin sections of the zeolites from the amygdaloid No. 535, 

 intended to bring to light the characters of the beautiful mineral which has been 

 widely distributed under the name thomsonite, and several sections of the species 

 lintonite, all bearing this number. These minerals are also enumerated under other 

 numbers (Nos. 163 A, 193 625 A, 625B) and by reference to those numbers further 

 facts may be found. These sections and others made of the same zeolite from other 



