CONCRETIONS. 



717 



and the largest concretion figured show beautifully that they took an exact 

 cast of the full thickness of a layer, since they are covered by raised undu- 

 lating ridges — the worm tracks mentioned on page 719. 



In the table below I have given: (1) an average of 18 analyses of 

 claystones from Massachusetts and Vermont, 5 from Geology of Massachu- 

 setts (p. 408), and 13 from Geology of Vermont (p. 700); (2) the propor- 

 tion of carbonates to the other constituents; (3) excluding the carbonates, 

 the remainder, reckoned to 100 per cent; and (4) the silica, alumina, and 

 water, reckoned to 100 per cent. 



Analyses of claystones from Massachusetts and Vermont, proportion of constituents, etc. 



CaCoj 

 MgCo 

 SiO. . 



A1,0:,. 



Fe:0:, 

 MnO, 



47.4 

 4.S 

 l<t. 1 

 10.9 

 6.1 

 1.9 

 4.3 



100 



39.6 



35.0 



12.7 



4.0 



8.7 



45.6 

 43.6 



100 



Column No. 2 indicates that a little more than half the mass of the 

 claystones is made up of carbonates, and these have been wholly or in large 

 part infiltrated, and this may be true also of much of the iron and man- 

 ganese. Column No. 4 would indicate that the original clays in which the 

 concretions used for analysis were formed were wholly kaolin ; indeed, there 

 is less silica and water than is needed for the formation of kaolin ; which 

 does not agree with the results of examinations with the microscope. The 

 analyses are mostly old, and, I have no doubt, give the quantity of the 

 AI0O3 too large, and that of the SiOo correspondingly small. 



Thin sections of the small, round and obovate claystones show no trace 

 of a continuous calcite cement binding the grains and needles of the clay 

 together, l)ut present a very fine and uniform mass of needles and grains, 

 not in any marked way distinguishable from the ordinary clay. When 

 treated with acids, eifervescence appears from spots covering the whole sur- 

 face, and the decalcified slide is made up almost wholly of kaolin needles, 

 mostly about 0.008 '"'° long and 0.002 '"■° wide, quartz grains 0.003 °"" across, 



