. Bulletin of the Geological Society of America. Geology. 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
. Bulletin of the Geological Society of America. Geology. 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^"^* %o FiouBE ^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 Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and
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