THE KEWEENAWAN SYSTEM 



335 



lava-flows and 5 conglomerate beds in succession, with neither top 

 nor bottom of the system exposed. 



The igneous rocks of the system consist principally of gabbros, 

 diabases, and porphyries; but other varieties are also present. The 

 si-dinu-ntary rocks, chiefly sandstone and conglomerate, were de- 

 rived largely from the igneous, 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; but there 

 is reason for questioning the interpretation of such figures. 



The total thickness of the Keweenawan system has been placed 

 as high as 50,000 feet. Interpreted in the simplest way, this would 

 seem to mean either that beds of rock were piled up nearly 10 miles 

 high on land, or that they filled a basin some 10 miles deep. Since 

 the upper part of the system is sedimentary, and sediments do not 



Fig. 307. Diagram of a series of beds formed on the abysmal slope of a conti- 

 nent, or in some similar situation, showing that the thickness, as usually measured, 

 ef, is not dependent on the depth of the basin, cd, and that a thick series does not 

 necessarily imply subsidence, even when the exposed portions of it show evidences of 

 shallow-water deposition at various horizons. 



accumulate in quantity in high places, the first of these suggestions 

 cannot be entertained, and it is extremely unlikely that there ever 

 was a basin 10 miles deep. 



The thickness of great bodies of stratified rock is commonly 

 measured as suggested by Fig. 307. The dip of the rock (p. 275) 

 and its extent at the surface are measured, and the depth is then 

 calculated on the principle that the thickness of the whole is equal 

 to the thickness of all its parts. The thicknesses of the several 

 beds, added together, is shown in the diagram by the line ef, whereas 

 the actual thickness, from top to bottom, is shown by the line cd. 



The point may be illustrated in another way. On the outer 

 slopes of continental shelves, and in deltas, sediments are laid down 

 with a considerable angle of slope. If the Amazon \u tv to build a 

 delta out 200 miles, the present ocean bottom remaining at an 



