SECRETARY'S REPORT. 115 



and this clay is studded all over, penetrated tlirongh in all ' 

 directions with beautiful crystals of gypsum, in the form that 

 mineralogists call selenite. The history of the wliole change is 

 at once revealed. The acid that still lingers in the upper clays, 

 is sulphuric acid in a very diluted form. The whole mass of 

 this clay has been slowly infiltrated from above by this diluted 

 acid, which has dissolved out all the shelly matter, forming 

 plaster of Paris from the carbonate of lime, which being 

 arrested by the impenetrable bed of clay below, has formed the 

 crystals of gypsum there. The farmer who has learned this 

 history knows where to find the gypsum deposit. When he 

 discovers the beds of clay from which the shells have been 

 dissolved, he feels pretty sure that some twenty or thirty feet 

 lower down he will find what is perhaps more valuable for his 

 grass land than the original marl would have been, for he knows 

 that in this region nature has manufactured the gypsum 

 through the destruction of the shells. 



We pass next to the belt lying west of this, in Pennsylvania, 

 New Jersey, Virginia, the Carolinas, and Georgia, and we come 

 upon the granites and gneissoid and slaty rocks, quite analogous 

 to those which we have here in the central and north-western 

 sections of New England. But in the region referred to, these 

 rocks have been decomposed to an extraordinary extent. The 

 soil there is nothing more, as it were, than effete rock, — the 

 residuum left by the decomposition of the strata beneath. 

 Tliere has been no soil transported from other districts and 

 spread over the surface. Therefore the study of the character 

 of the rock gives, in a large degree, a clue to the character of 

 the soil. So deep has been this process of decomposition, 

 through the slow agency of rain descending upon the surface 

 and the oxygen of the atmosphere attacking the various metallic 

 compounds, that in many places where the apprehension was 

 felt that enormous rock cuttings would become necessary in the 

 construction of railroads, it has proved that the pick and spade 

 were all that was required ; and yet, so free from violence has 

 been the change that the very structure 'of the mica-slates and 

 the talc-slates is perfectly preserved. You may walk through a 

 deep cut in one of those railroads, where the wall may be as 

 much as sixty or seventy feet high on either side, and you will 

 think you see solid rock, consisting of colored bands of mica- 



