Q 



64 A. C. LANE WEIGHT OF SEDIMENTARY ROCKS 



greater porosity. But, in the first place, they give a sample of material 

 scraped over by the glacier, from a considerable surface, and are a rock 

 flour which approaches close to the average rock, as estimated by Clarke, 

 Daly, and Washington, as the analyses given below show. In the second 

 place, they are firm enough, perhaps were even loaded down by ice, to 

 hold their shape wet or dry long enough to be measured, either by volume 

 or by paraffin dip. So I have had students and assistants help me in 

 making repeated tests on the clay of the clay pits of the New England 

 Brick Company of Medford, near the Wellington Bridge, and on River- 

 side Avenue, about one mile from Medford Square. They were also the 

 subject of a graduation thesis by L. H. Carter in 1912. The pits are 

 about 30 feet deep. Under 5 to 6 feet of outwash gravel comes a 10-foot 

 layer of clay; then 1 to 2 feet of gravel; then another clay bed. Carter's 

 analysis is of the lower bed. 



Medford Brick-clay 



L. H. Carter, Analyst 



Average of Three Analyses 



With moisture. Moisture-froo. 



Silica 57.60 60.7 



Alumina 19 . 09 20.06 



Ferric oxide 2.55 2.69 



Ferrous oxide 4 . 20 4 . 40 



Magnesia 5 . 19 3 . 26 



Lime 2 . 12 2 . 24 



Soda 1.40 1.5 



Potash 4 . 60 4. 86 



Water 5 .06 



P 2 5 40 .42 



S 16 .17 



Titanium oxide, present with alumina, about 1 per cent. 



100.36 100.30 



Chlorine, S. A. Siegel, .047. 



Average weight of same material as Mr. Carter analyzed, air-dried seven 

 years (shrinkage cracked) = G d , 1.71 ± .013. 



Average weight of samples wet from the bank, upper stratum = G .. 

 2.08 ± .017. 



Samples taken in a series of distances in from the surface of the bank gave 

 intermediate values from 1.74 to 1.9+ . 



Total water, one sample, 15.77 per cent. 



Loss on ignition, 2.82 per cent. 



There is marked shrinkage in drying, so that D < G. and is < 1.71. 



Thus porosity = G w — D > .374-. 



Whereas in the average sample tested the porosity would be if G =2.8, not 

 over .344. 



