THE PROTEROZOIC ERA 461 



In the valley of the St. Croix River, in northwestern Wisconsin 

 and the adjacent parts of Minnesota, there are said to be 65 lava- 

 flows and 5 conglomerate beds in succession, with neither top nor 

 bottom of the system exposed. 1 Some of the igneous rocks, such 

 as the gabbros of the Vermilion 2 and Mesabi 3 regions, are intrusive 

 (laccolithic) . The laccolithic intrusions of the Mesabi region have 

 occasioned extensive metamorphism in the rocks into which they 

 were intruded. The igneous rocks of the system consist principally 

 of gabbros, diabases, and porphyries; but other varieties are also 

 present. 



The sedimentary rocks of the system were derived largely from 

 the igneous rock, and their character is such as to indicate that 

 they accumulated rapidly. The thickness of the sedimentary beds 

 has been estimated at some 15,000 feet. 



Estimated thicknesses. It is important to note the meaning of 

 the great thicknesses sometimes assigned to sedimentary and 

 igneous formations. In the case of the Keweenawan, for example, 

 the thickness of igneous and sedimentary rocks has been placed as 

 high as 50,000 feet, or nearly ten miles. When great thicknesses 

 of sedimentary rock show signs of deposition in shallow water (or 

 perhaps on land), as in this case, it is commonly inferred that the 

 bottom of the basin of deposition sank during the deposition to an 

 amount corresponding approximately to the thicknesses. In 

 some cases, the amount of sinking assumed exceeds the greatest depth 

 of the oceans. Moreover, if sedimentary rocks accumulated to the 

 thickness assumed, their lower parts would be down where high 

 temperatures prevail, and where the pressure would be such that 

 all crevices and pores would be obliterated, and the rock highly 

 metamorphosed. As a result of uplift and erosion the deeper parts 

 of these thick systems are sometimes exposed, and in many cases 

 do not show the effects of great heat and pressure. In view of 

 these considerations, two positions have been taken by geologists. 

 (1) The correctness of the estimated thicknesses has been ques- 

 tioned, or (2), accepting the estimates, far-reaching theories of 



1 Hall, BuU. Geol. Soc. Am., Vol. XII, pp. 313-340. 



2 Clements, Mono. XLV, U. S. Geol. Surv. 

 3 Leith, Mono. XLIII, U. S. Geol. Surv. 



