THE CHAHPLAIjST CLAYS 



323 



during seasons of rainfall, as to be quite unsuited for cultivation. 

 Mixed with varying proportions of siliceous sand to counteract 

 shrinkage, they form the common brick-making materials of the 

 Northeastern states. 



The materials of the Leda clays naturally vary in different 

 localities, being dependent on the characteristics of the rocks 

 from which they were de- 

 rived. Those of Canada, 

 according to Dawson, 

 were derived from the 

 waste of the Utica and 

 Quebec groups. This 

 authority believes that 

 when the clay was in sus- 

 pension, it was probably 

 of a reddish or brown 

 color from the iron per- 

 oxide it contained, but 

 that, like the bottom mud 

 now forming in the deeper 

 parts of the St. Lawrence, 

 the coloring matter be- Fig. 34. — Showing particles from Leda 

 came deoxidized by or- ^lajs. ^l, quartz j 2, orthoclasej 3, plagio- 



ganic matter so soon as 



deposited, the sesquioxide 



of iron being converted into sulphide or protoxide carbonate. 



Inasmuch, however, as the materials were so largely derived 



t)y the grinding action of the glaciers on fresh rocks, it is not 



impossible that they may have been again deposited as clay 



without having ever undergone the oxidizing process. 



Unlike the till or boulder clays, these Leda clays are dis- 

 tinctly stratified, as shown in the accompanying illustration. (PL 

 27.) An analysis of a sample from the locality figured yielded 

 the author results as given in column I on p. 324. In column II 

 is given that of the portion (33.56%) soluble in hydrochloric 

 acid and sodium carbonate solutions, while in column III is given 

 the composition of a ** semi-assorted glacio-lacustrine'' clay 

 bordering on Lake Michigan near Milwaukee, "Wisconsin, and 

 in IV a glacial pebbly clay underlying II at the same locality.^ 



* Analyses II and III from Chamberlin and Salisbury's paper, 6tli Ann. 

 Kep. U. S. Geol. Survey, 1884-85. 



elase; 4, mieaj 4, tourmaline ; 6, pyroxene; 

 7, cMorite; 8, hornblende. 



