362 A. ('. LANE WEIGHT OF SEDIMENTARY ROCKS 



at the densest part, a little below the center, growing then less to the 

 bottom. I add a few other drill-core results from Keewenawan rocks. It 

 will be seen that the variation from Moore's results is less than 5 per cent. 



Weight dry. in tons 

 . per cubic meter. 



Sedimentary: 

 Eastern sandstone, closely resembling Moore's 

 Triassic sandstone, but older and deeper and 



near fault ( ( 'ambrian ) , Oneco S . 1270+ 2 . 35 



Oneco. 8.1850 2.50 



Oneco 8.715 2.62 



Fine-grained Sediment: 



Nonesuch shale, black, near lode and fault, with small vein 2.80 



Arvon (Huronian) black slate 2.78 



Igneous Rocks: 



Porphyry with lithophysae, Onondaga 8.700 2.42 



Porous porphyry. Onondaga 7.177 2.60 



lihyolitic felsite, Onondaga 7 . 550 2 . 72 



Quartz porphyry, Onondaga 8. 894 2 . 41 



Ophite, dense basalt like Causeway basalt or 



Hendre dolerite, Onondaga 4.141 2.91 



Same as above, North Lake 3.110 2.93 



Same as above, Oneco 9.660 2.96 



Data ox friable Material 



It will be seen, however, that Moore's tests do not give much informa- 

 tion regarding the friable shales and clays and the unconsolidated strata. 

 It would be natural for them to be more porous, and hence lighter in 

 tons per cubic meter than the more coherent rocks. The anomalies or 

 variations in observed gravity from that computed 12 under the assump- 

 tion of a 60-kilometer crust down to the level of hydrostatic pressure (the 

 isostatic or asthenic layer) seem in regions known to me surely, and are 

 probably generally, due to variations in the weight per cubic foot of the 

 upper layers. It is important, then, to know what is the weight per 

 cubic foot of the sedimentary rocks which are so often the outer layers. 

 Deviations from this might mean extra light rocks filled with gas or 

 gypsum, or also extra heavy intrusions of trap. As to the so-called 

 "marls/' really bog-limes, the limestone dough of fresh-water limestones, 

 we get some information from the estimates of the Portland cement fac- 

 tories as to the amount required to make a barrel of cement. 



For instance, from the Michigan report on marl 1 - we may calculate 



'-Bowie: Coast and Geodetic Survey papers, especially no. -lo. 

 1:! Michigan Geological Survey, vol. viii. part iii. pp. 1G7-S. 



