192 GEOLOGY. 



time give some idea of the deformation and disruption to which the 

 Huronian rocks have been subject. 



The Keweenawan system. 



Constitution and thickness. — Overlying the Animikean system un- 

 conformably, there is, in some parts of the Lake Superior region, a 

 third series of pre-Cambrian rocks, the Keweenawan. 1 A map of 

 Keweenaw Point (N. Mich.) is shown in Fig. 75, and a section of the 

 Keweenawan system in Fig. 76. The maximum thickness of the system 

 has been estimated at nearly 50,000 feet, but it is not impossible that 

 this estimate is an exaggerated one. If it be correct, the Keweenawan 

 is the thickest body of post-Archean rock referred to any one period. 

 This seemingly incredible thickness may merely mean inclined depo- 

 sition, with subsequent tilting and shearing, as explained on p. 262, 

 and the estimate be altogether correct. 



The lower and thicker portion of the system is made up chiefly of 

 igneous rock, representing a succession of lava flows and intrusions, 

 while the upper part is chiefly sedimentary. The earlier flows of lava 

 seem to have occurred on the land, and to have succeeded one another 

 at short intervals, for the surface of one flow was not sensibly modified 

 by surface agencies before the next overspread it. In places, the scori- 

 aceous surface of a flow is still preserved beneath that next succeeding, 

 and serves to distinguish one lava flow from another. Later in the 

 period, the intervals between the flows of lava appear to have become 

 longer, and thin beds of sediment were deposited between successive 

 sheets of igneous rock. Still higher in the series, the sedimentary 

 beds increase in importance, while the thickness of the intercalated 

 lava beds becomes less, until, in the upper part of the system, the 

 lava beds fail altogether, and there follows a succession of sandstones 

 and conglomerates having an estimated maximum thickness of some- 

 thing like 15,000 feet. 



The igneous rocks of the system consist principally of gabbros, dia- 

 bases, and porphyries, but other varieties are also present, and the range 

 is greater in the upper part of the igneous division than in the lower. 



The sedimentary rocks associated with the igneous and lying above 



1 A general account of this series is given in Vol. I, Geology of Wisconsin, 1881; 

 see also other Wisconsin and Michigan reports, and Mono. V, U. S. Geol. Surv. 



