Association of American Geologists. 159 
Mr. Boyé enquired whether the antacid powers of magnesia, 
and its effects on vegetation, had been noticed, as it exists in the 
dolomitic limestone. - 
Dr. Jackson inferred from his observations, that magnesia is 
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 muck it oe 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 quantity 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: 
D. Rogers, thought that some of the marls of New 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- 
Hon be useful in agriculture. 
The subject of potash in soils was next brought. before the 
meeting. Dr. Jackson inquired if any experiments had been 
