268 STUDIES FOR STUDENTS 



would be evident upon a complete analysis of the alluvium, but unfor- 

 tunately for geological purposes such complete analyses of soils are 

 seldom made. Lime exists in higher percentage in river alluvium 

 than in upland soils, but Mann has shown that it is extremely deficient 

 in the soils subjected to heavy rainfall, the general average in the Assam 

 alluvium soluble in hydrochloric acid being about 0.08 per cent., as 

 against nearly i.o per cent, in the average Indo-Gangetic soil.' It 

 is noticeable that in the true tropical soils the content of magnesia is 

 considerably above that of lime; a fact readily intelligible from the 

 more ready solubility of lime in carbonated water. ^ That it is leached 

 out also, however, is indicated by its content of o . 5 per cent, in the 

 soluble portion of the Assam tea soils as contrasted with its presence to 

 the extent of i .3 per cent, in the soluble portion of the Indo-Gangetic 

 alluvium.3 The iron of the Assam soils is also low, but it is not 

 deficient in tropical soils in general, giving on the contrary a character- 

 istic red to such lands as Madagascar and Ceylon. 



The continuously rainy climates may be divided into those situated 

 in the equatorial belt, usually possessing at least short dry seasons, 

 and those situated in the cooler parts of the temperate zones. The 

 preceding discussion on the chemical distinctions has been largely 

 based upon soils of the torrid zone or warm temperate, as in the case 

 of the Assam alluvium. In the cold temperate regions recent glacia- 

 tion has in many cases prevented the establishment of normal chemical 

 relations between the rocks and the atmosphere, but such observations 

 as have been, made indicate, as was shown in Part I, that decom- 

 position is greatly reduced. From the foregoing it may be concluded 

 that the broad association of carbon with sediments which are thor- 

 oughly decomposed and leached throughout is the mark of coatinu- 

 ously rainy climates which are tropic or at least warm temperate; 

 with sediments imperfectly decomposed and incompletely leached 

 the mark of more or less continuously rainy climates which are in 

 addition cool or cold. The best microscopic test after the lithification 

 of the alluvium may be the absence or presence of potash minerals. 

 The carbonaceous shales of the anthracite coal-measures of Pennsyl- 

 vania, except the fire clays immediately below the coal beds, possess a 

 marked abundance of muscovite, indicating the presence of consider- 



I Hilgard, op. cit., p. 413. ^ Qp^ cit., p. 405. 3 Op. cit., p. 413. 



