153 



nature was, he said, well exemplified in the tertiary clays and 

 sands of extensive districts in the marl region of Maryland and 

 Virginia. Some of these deposits contain, even now, so much 

 free or only partially neutralized sulphuric acid as to impart to 

 the mass a strong acid flavor and reaction. The shells originally 

 imbedded in them having been entirely removed by the solvent 

 action of the acid, have left in the clay and sand innumerable 

 hollow casts, beautifully impressed with the external markings of 

 the fossils, while the sulphate of lime formed by this reaction 

 has been accumulated often in large groups of selenite crystals 

 in the lower and comparatively impervious layers. In many 

 cases, the residuary sulphuric acid, in the presence of much dif- 

 fused organic matter, has been wholly deprived of its oxygen, 

 and reduced to the condition of sulphur, so that, over wide areas, 

 these ancient, and now effete marl beds, exhale a sulphurous 

 odor, and yield a sensible amount of sulphur when exposed to 

 heat in a close vessel. 



Prof. Rogers then referred to the acid and alkaline springs of 

 th^ate formations of the Appalachian belt, as depending on 

 the development and reactions of sulphuric acid. The former 

 class of springs, always containing an excess of this acid, along 

 with earthy and other sulphates, were observed to originate in 

 belts of slaty rock, containing no carbonate of lime, either dif- 

 fused or in the shape of interpolated layers. The latter issued 

 from slates rendered more or less calcareous by the presence of 

 fossil shells and plates of limestone. The sulphuric acid evolved 

 by the oxidation of the iron pyrites abounding in both these vari- 

 eties of slate remained, in the former case, in part uncarbonized, 

 and was carried off by the waters of the so called Alum Springs; 

 but in the calcareous slates the effect was different. Here, re- 

 acting with the carbonate of lime, it became neutralized, and, at 

 the same time, set free an immense amount of carbonic acid. 

 This agent, favored probably by pressure due to the depth, as 

 well as by relative quantity, dissolved out a portion of the car- 

 bonates of lime and magnesia of the slates, and by reacting on 

 the salts of soda, always present, formed the carbonate of that 

 base. The percolating water thus became impregnated with 

 earthy and alkaline carbonates, and with an unusual amount of 



