Association of American Geologists. 159 



Mr. Boye enquired whether the antacid powers of magnesia, 

 and its eflects on vegetation, had been noticed, as it exists in the 

 dolomitic Hmestone. 



Dr. Jackson inferred from his observations, that magnesia ie 

 injurious only when used in a caustic state, in the same manner as 

 caustic lime is known to be injurious to vegetation by abstract- 

 ing carbonic acid from the atmosphere, and from decomposing 

 vegetable and animal matters. It also acts unfavorably in virtue 

 of its hydraulic power ; rendering in some cases, the soil very 

 hard. He further stated, that when composted with peat and 

 swamp nuick it gained from these substances phosphoric acid, 

 and thus became the means of conveying to wheat and other ce- 

 real grains the phosphate of magnesia, known to be always found 

 in their ashes. Dr. Jackson further considered the combinations 

 of lime with the organic acids of soils as deserving much atten- 

 tion. He had found subsoils to contain a larger quanfity of cre- 

 nates of lime than the soil, and that some streams in like manner 

 contained a larger proportion of soluble crenates than others, and 

 these former are most valuable for the purposes of irrigation. 

 Dr. J. recommends the employment of a compost of lime, swamp 

 muck or peat, and animal manure, and he attributes the benefi- 

 cial effect of this in part to the evolution of ammonia consequent 

 upon the decomposition of the organic matters. 



The discussion then turned on the character of hydrated perox- 

 ide of iron on vegetation. It was thought by Dr. Jackson, that 

 the injurious effects sometimes known to arise from it were to be 

 attributed to free sulphuric acid contained in it from the decom- 

 position of sulphuret of iron. A marl was cited which at first 

 produced very luxuriant vegetation, but at a subsequent period 

 was found to destroy the plants growing where it had been used, 

 owing to the decomposition of sulphuret of iron contained in it, 

 producing free sulphuric acid, which corroded the plants. Prof. 

 H. D. Rogers, thought that some of the marls of JVew Jersey, 

 contained so much sulphuret of iron as to require more alkaline 

 matter than was to be found in them to neutralize the acid result- 

 ing from their decomposition. Still it was thought that small 

 quantities of sulphuret of iron, in a marl, would by its decomposi- 

 tion be useful in agriculture. 



The subject of potEish in soils was next brought before the 

 meeting. Dr. Jackson inquired if any experiments had been 



