328 C. W. HALL — KEWEENAWAN AREA OF EASTERN MINNESOTA 



minute spherical cavities to those tlie size of marrowfat peas. Occa- 

 sioiiall}' some are hirger and vary greatly in shape from the usual spherical 

 and elliptical form. All are filled, or partially filled, with secondary 

 minerals. Laumonite seems to be the principal one, w'hile calcite is 

 abundant. Locally the cavities have a lining of quartz attached to the 

 w^all in a crystalline fihn from which project toward the center of the 

 cavit}^ num])erless })yramids of quartz cr3'^stals. These cavities no*v 

 appear to be only partially filled. 



The flows, of which the foregoing is a generally applicable description, 

 extend in continuous superposition down the river for nearl}^ 3 miles, 

 those at the dam being the bottom of the visible series. There is great 







oO^"^* 



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FiouBE l.-^Sketch across a typical Lava Flow, Chengwatana Series. 

 This flow is 30 feet thick and lies between two conglomerate beds. 



diversity in thickness. In fact, in the entire distance there are no two 

 alike, either in total thickness or in the proportions of the compact and 

 amygdaloidal parts into which every flow can be subdivided. The 

 thinnest flow is not more than 8 feet in thickness, yet it presents all the 

 characters of the thicker ones. From this thinness all measurements 

 are noted up to 200 feet, the thickest flow seen in the ser^s. 



Associated wdth the foregoing structural characters are still others. 

 Some of the beds display a conspicuous tendency to concentric weather- 

 ing. As the blocks separate through temperature changes, and particu- 

 larly freezing, the angles and edges disappear ra})idly, and the material 

 cracks and peels off' in concentric layers which var}^ in thickness to some 

 extent with the size of the blocks sufi'ering disintegration. 



