400 TESTIMONY IN OPPOSITION. 



view to their proportions of calcareous matter. But proportions 

 so large as 40, 30, or even 20 per cent, of green-sand could not 

 thus escape even careless and superficial observation ; for even the 

 smallest of these proportions would give a very manifest greenish 

 or gray tint to any otherwise light-coloured marl. Knowing the 

 great uncertainty of the guessings at proportions of green-sand 

 naturally intermixed with marl or other earth, I did not rely on 

 them except as to the absence of any very large proportion. For 

 more accurate testing, the clayey parts were washed off in water ; 

 in others the calcareous parts were also removed by weak acid. 

 And for still better means of judging by comparison, I mixed toge- 

 ther, in different and known proportions, measured quantities of 

 light-coloured marl (such as are all those about Williamsburg), and 

 pure green-sand prepared by washing some obtained from the 

 richest beds in New Jersey. And of such artificial compounds, 

 examined by the eye both when dry and in powder, and wet, and also 

 after being again dried in mass, the admixture of green-sand, even 

 when as small as 10 per cent., was obviously more abundant than 

 in the miocene marls reputed to be among the richest in green-sand. 

 Under these circumstances, without denying the possible existence 

 of such cases, it is proper to wait for and to require further proofs 

 of assertions of such large proportions as 20 to 40 per cent. 



But there is much better support for my position, of the general 

 scarcity of green-sand in miocene marls, than any proofs, positive 

 or negative, that I can adduce, presented by Prof. Rogers himself, 

 in his " Report of the Progress of the Geological Survey'' for 1837. 

 He therein gives a tabular statement of 148 specimens selected by 

 his assistants, and their analyses made under his own direction. It 

 is to be presumed that so many specimens, and thus obtained, must 

 present a fair and correct average of general quality of the marls 

 of the region in which they were found ; or at least that their con- 

 tents would not be too little favourable to the geologist's preconceiv- 

 ed opinions, or assertions. The specimens were from eighteen coun- 

 ties, viz. : Lancaster, "Westmoreland, Richmond, Northumberland, 

 King G-eorge, Mathews, Middlesex, Gloucester, King and Queen, 

 King William, Essex, Isle of Wight, Nansemond, Elizabeth City, 

 Surry, Prince George, James City, and Warwick. Of these 148 

 specimens, of one only (S. Downing' s, Lancaster) is the quantity 

 or proportion of green-sand stated with any approach to precision. 

 This is said (no doubt by guess) to contain " 10 or 12 per cent, of 

 green-sand," and only 17 per cent, of carbonate of lime. Of five 

 others, the green-sand would seem to be in notable quantities, but 

 as no numbers or proportions are named, it may be inferred that 

 the proportions were deemed less than the one just stated. These 

 five are described as follows, in regard to this ingredient : Calla- 

 han's, Lancaster, "large grains of green-sand in considerable quan- 



