176 GEOLOGY. 



their metamorphosed products, a subordinate amount of limestone, 

 and beds of iron ore derived by alteration from beds of ferruginous 

 (non-clastic) sediments. The beds of limestone (dolomite) are not 

 known to contain fossils, and their organic origin cannot be affirmed, 

 though this is perhaps their most reasonable explanation. 



The period of Huronian sedimentation was long, for in areas favor- 

 ably situated thicknesses of sediment to be measured by thousands of 

 feet rather than by denominations of a lower order, were deposited. 

 Thicknesses of 6000 feet, and perhaps of 18,000 (see p. 181), are re- 

 ported. That sedimentation was not uniform throughout the region 

 is indicated both by the fact that the number of formations included 

 in the system is different in different regions, and by the fact that 

 the thickness of a given formation sometimes varies greatly within 

 short distances. In the Menominee region there are five sedimentary 

 formations, in the Marquette region nine, while north of Lake Huron 

 thirteen 1 have been enumerated. The lesser number at some points 

 may be in part the result of erosion which has carried away a portion 

 of the system, but it is also probable that the number of formations 

 did not originally correspond in different parts of the broad area under 

 consideration. 



The sedimentary beds of the Huronian system are cut by dikes and 

 affected by igneous intrusions of great and small extent. From the 

 relations of the igneous rocks to the sedimentary, it is clear that 

 extrusions of lava and deposits of volcanic elastics took place at 

 various times during the period, the lavas and tuffs being covered in 

 turn by sediments. Where the metamorphism has been great, it is 

 not now possible in all cases to say whether a given body of igneous 

 rock was intrusive or extrusive, and in many cases where it was in- 

 trusive, the date of irruption cannot be fixed. Locally, and even for 



Fig. 60. — Section of the Proterozoic in the Marquette region, N. Mich. Aln, Ne- 

 gaunee (iron-bearing) formation (Huronian); Aui, Ishpeming formation (Animi- 

 kean); Aed, eruptive diabase or diorite. Length of section, 3 miles. (Van Hise, 

 Bayley, Smyth, Mono. XXVIII, U. S. Geol. Surv.) 



considerable areas, the igneous rocks are more abundant than the 

 sedimentary in the exposed part of the system (Fig. 60). 



1 Seep. 181, and foot-note 3, p. 151. 



