212 



observations go to show that the rather dark green variety is 

 more potent in its effects than the very hght green, which some- 

 times overlies it. 



The presence or absence of shells I look upon to be a point of 

 but little moment, for I find that several of the most active marls 

 in the region show no traces of fossils. The whole amount of 

 carbonate of lime in the shape of fossils, and in that of the occa- 

 sional white incrustation upon the grains, can in very few in- 

 stances amount to one per cent; while, as analysis shows, the 

 lime chemically combined with the other ingredients in the green 

 grains, is sometimes one per cent., and the potash nearly twelve 

 per cent. 



There yet remains, however, a more important, and by far 

 more difficult inquiry, namely — into the exact constitution of the 

 green grains, in order to determine the per-centage of the several 

 ingredients — or the richness of the marl in potash and lime. I 

 had entertained hopes that the external aspect of the grains 

 might perhaps depend in part on the presence and proportion 

 of these bodies, and that mere inspection, after multiplied ana- 

 lyses were made, might enable any one following certain direc- 

 tions, to inform himself whether a marl abounded in these essen- 

 tials or not. But, I find that so far from being a mineral of 

 definite and constant proportions, as spme mineralogists have 

 regarded it, the greensand is in fact a compound which fluctuates 

 considerably in its external characters, and its chemical compo- 

 sition. The numerous analytical results which I have presented, 

 will abundantly prove this. It is manifest, therefore, that we 

 possess no shorter method to discover the exact quantity of the 

 potash, than to subject the marl to a systematic chemical analysis. 



Three different plans have been pursued in procuring the com- 

 position of the marls described in this work. The^rsf consisted 

 in separating, mechanically, that is to say, by washing and 

 selecting the several ingredients, the greensand, the clay, and the 

 quartzose sand, together with any carbonate of lime which may 

 be present. The second, in analyzing systematically the green 

 granular mineral, with a view to determine accurately its consti- 

 tution. The third, in partially analyzing the marl for the purpose 

 of ascertaining the proportion of its chiefly efficacious element, 

 the potash. Besides determining the relative quantity of potash 



