EXERCISE II. 

 THE ORIGIN OF SOILS. 



Supplies for a Laboratory Section of Twelve. Six specimens of granite containing large grains of the min- 

 erals quartz, feldspar, mica, and hornblende. Six each of labeled specimens of quartz, feldspar, mica and horn- 

 blende. At least six specimens of each of the following: Fresh granite, weathered granite, sandstones (including 

 specimens of quartzite), limestones and shales. One iron mortar for crushing small bits of rock specimens. Twelve 

 glass plates, 4"x4". Twelve reagent bottles tilled with hydrochloric acid ; coarse sand ; silt loam ; twelve glass 

 tubes ; basin. 



Part A. The Weathering of Some Important Soil-Forming Rocks- 



1. Examine very closely the crystals of which a piece of granite is composed, (a) Iden- 

 tify and give names to the various substances which enter into the composition of the piece 

 of granite which you have at hand, (b) Why would it be incorrect to call granite a mineral? 

 (c) What name may be applied to such substances as granite in order to distinguish them 

 from substances which, strictly speaking, are minerals? * 



2. Granite is composed essentially of feldspar and quartz associated with other minerals. 

 It is common to many regions where great movements of the earth's crust have taken place 

 as, for example, in the Rocky Mountains, Black Hills, and New England, (a) From your 

 study of quartz and feldspar, which of the two minerals do you infer will weather more read- 

 ily? (b) Observe a specimen of weathered granite. What mineral shows the greatest re- 

 sistance? (c) What would be the nature of the soil formed from this more resistant min- 

 eral; i. e., would it be sandy or clayey? 



3. As granite and similar rocks decompose they give rise to a mixture of coarse and 

 fine particles. (It has been arbitrarily decided that the various classes of soil shall be named 

 according to the sizes of particles of which they are composed. For example, soils composed 

 almost wholly of large particles are called gravel or sand. Soils composed almost wholly of 

 very fine particles are called silt. While soils containing a large percent of extremely fine 

 particles are classed as clay. In case no particular size of particles predominates in a soil 

 it is called a loam.) As grains of sand and finer material disintegrate from rocks they are 

 readily sorted by running water and may sooner or later be deposited as beds of sand, beds 

 of silt, and beds of clay. The water of streams and fresh water lakes contains some mineral 

 matter in solution. Oceans and salt lakes are dense solutions of mineral water. As such 

 water percolates through beds of sand, it usually deposits some of the mineral matter con- 

 tained in solution about the sand particles, thus cementing them together. The resulting ma- 

 terial is then spoken of as sandstone. The durability of sandstone depends much upon the 

 nature of the cementing material. 



Observe the specimens at hand. (Quartzite, a much modified sandstone, has undergone 

 more of the effects of moderate heat, chemical action and pressure than has the ordinary sand- 

 stone, which fact explains its different appearance.) (a) From which specimens can you 

 loosen grains most readily? (b) Test the porosity of these specimens, (c) Describe the vari- 

 ous specimens of sandstone which you have at hand, as to hardness, porosity, and strength of 

 cementing material, (d) The weathering of sandstone would give rise to what kind of 

 soil? 



4. Limestones vary greatly on account of differences in origin and kinds of impurities 

 present. Some contain much clayey matter, others much sand, while a few may be classed 

 as almost pure limestone, (a) What is the color of the specimen at hand? (b) Hardness? 

 (c) Porosity? (d) Place a small amount of crushed limestone in a test tube. Add hydro- 



* Note — Many rocks are made up of a number of minerals, as you have observed in the case of granite. A 

 number of important soil-forming rocks, however, are almost wholly made up of but one or two minerals, for ex- 

 ample, limestone and sandstone. 



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