322 TERTIARY GREEN SAND. 



experienced upon them from the use of gypsum. A 

 single bushel an acre, applied to these red clays, was said 

 by Dr Higgins to produce quite a wonderful effect. 

 Careful analyses of repeated specimens of these soils 

 might throw some light upon the still obscure source of 

 the virtues of this mineral substance in promoting vege 

 tation. In Pennsylvania, I was informed that, as in 

 western New York, its good effects were most observable 

 on limestone soils. 



Jan. 29. At 9 A.M. I left Baltimore for Washington, 

 where I arrived in less than two hours. The road ran 

 very much through the tertiary sands, in which, where 

 cuttings occurred, occasional beds of decided green sand 

 were seen, the chemical nature of which it would be 

 interesting to compare with that of the similar coloured 

 sands beneath the chalk. These tertiary sands are very 

 generally ferruginous, and, in the hills around Baltimore, 

 the beds of clay which occur interstratified with the 

 sand are rich in nodules of hematite and carbonate of 

 iron, which are mined for and smelted in the neighbour 

 hood. The production of these nodules, which is a con 

 sequence of the presence of layers of clay capable of 

 arresting the ferruginous waters descending from above, 

 is an interesting fact both in a chemical and an economi 

 cal point of view. It has much similarity, also, both as 

 a fact and in its cause, to the occurrence of clay ironstone 

 nodules, and of beds of carbonate of iron, among the 

 shaly beds of our coal-measures. 



In Washington I took up my quarters with an old 

 friend, Dr Henry, formerly of Princetown University, 

 well known in England for his electro-magnetic and 

 other physical researches, and now secretary to the 

 Smithsonian Institution at Washington. 



