36 SOILS. 



composes more readily, and the rocks in which it mainly occurs 

 (hydromica schists) are refractory to weathering, and in any 

 case do not yield soils of any fertility, the mineral being as- 

 sociated simply with quartz. 



Chlorite, essentially a silicate of alumina and iron, somewhat 

 resembles mica but is deep green or black, in small scales. It 

 forms part of certain rocks (chlorite schists), which greatly 

 resemble the hornblende schists, but are usually inferior to the 

 latter as soil-formers, containing but little of any direct value to 

 plant life. 



Talc and Serpentine, Hydrous silicates of magnesia, are 

 extensive rock-materials in some regions, and as such require 

 mention as soil-formers also. Serpentine usually forms black- 

 ish-green rock-masses, that although soft disintegrate very 

 slowly in the absence of definite structure, and are attacked 

 with some energy only when charged as is frequently the 

 case with ferrous oxide. The conversion of this into ferric 

 hydrate, so common in nature, here also serves as the point of 

 attack on a rock otherwise very stable; causing it to crumble, 

 even though slowly. 



Talc (the true "soapstone") being usually free from iron, 

 would be even more slow than serpentine to yield to weather- 

 ing, but that its extreme softness and ready cleavage greatly 

 facilitate its abrasion. Thus talc schist, which is usually a 

 mixture of talc with more or less quartz, undergoes mechanical 

 disintegration quite readily. 



But the soils formed from either serpentine or talcose rocks 

 are almost always very poor in plant food, and sometimes 

 totally sterile. Magnesia, though an indispensable ingredient 

 of plant food, is rarely deficient in soils and unlike lime does 

 not influence in any sensible degree the process of soil forma- 

 tion. Magnesian rocks as a whole are practically found to be 

 not specially desirable soil-formers, even in the form of 

 magnesian limestones. They do not even, as a rule, contain 

 as many useful accessory minerals as are commonly found in 

 limestones. Moreover, an excess of magnesia over lime is 

 injurious to most crops, as is shown later (chapt. 18). 



The Zeolites. Zeolites may be defined as hydro-silicates 

 containing as bases chiefly lime and alumina, commonly to- 



