124 



INSTITUTE OF SOCIAL ANTHROPOLOGY PUBLICATION NO. 1 1 



bad when I was a small boy" (some 70 years ago). 

 The majority opinion was that most of the slopes 

 had been denuded and gullied for more than 50 

 years. Probably erosion has been excessive ever 

 since the chaotic period 1856-76. Other large 

 areas stripped of soil and much gullied include 

 the margins of the Mesa de Atzimbo, the slopes 

 above Carfngaro, the slopes back of Zirandan- 

 gacho, the valley of the Arroyo del Sal to above the 

 Puerta de Cuenembo, and the slopes between El 

 Tigre and Sanambo. Recent channeling and 

 gullying has taken place in many areas of gentler 

 slopes such as the Atzimbo-La Noria area above 

 the falls of the Arroyo del Sal to, in the area north- 

 west of Quiroga drained by branches of the 

 Arroyo de Quiroga, in the Tirimicua Basin, and 

 in the Icuacato Basin. 



Through erosion and denudation, probably half 

 of the arable land of Quiroga has been removed 

 from cultivation. The common resultant surface 

 is composed of bare volcanic rock and tepetate. 

 The word tepetate requires some discussion since 

 it occurs frequently in local speech as well as in 

 the literature of Mexican geology and pedology. 

 It seems to have come from two Mexicano words 

 {tetl—Tock+petlatl=m&t) meaning "rock-mat" or 

 nature's flooring, or possibly from tepetl=hil\-\- 

 petlatl=mat. By some geologists it has been 

 restricted to such meanings as: andesitic tuff; 

 tuffaceous conglomerates, pumaceous brecchias, 

 pumaceous stone; all nonmetallic earth and ore 

 from mines; a solid layer of the earth cut like 

 peat or quarried rock for house construction; a 

 yellowish-white clayey substance or sott stone; 

 rotten or decomposed rock; the impermeable 

 horizon C often revealed by erosion; etc. In the 

 Quiroga area this Mexicano term is widely used, 

 and several localities are known as Los Tepetates. 

 Apparently, in Quiroga at least, tepetate is applied 

 to the impermeable soil horizon C or subsoil which 

 is normally ^2 to IK meters beneath the surface — 

 but veiy frequently exposed by denudation and 

 erosion. Although it is often found developing 

 from tuffs and brecchias, apparently any vol- 

 canic mass that has begun to change into hard 

 compact clay can be termed tepetate. The color 

 varies from brick red to whitish yellow, but reds 

 and browns are most common. It is the normally 

 exposed "subsoil" on the slopes, and is perfectly 

 useless for cultivation. In time it probably 



weathers into the tierra charanda, a red ferruginous 

 clay. 



The rocks of the area are about 99 percent 

 silicates. The feldspars or silicates of aluminum 

 comprise about 68 percent, and olivine, amphi- 

 boles, and pyroxenes constitute nearly 32 percent 

 by weight of the rocks. Plagioclase makes up 

 about six-sevenths of the feldspars, the remainder 

 being orthoclase. Calciclase (labradorite and by- 

 townite predominantly) constitutes more than 

 two-thirds of the plagioclase, the sodaclase being 

 principally andesine. The next important in- 

 gredient is magnesium silicate or olivine. There 

 are relatively small quantities of diopside, hypers- 

 thene, bronzite, augite, and hornblende. Various 

 iron oxides (magnetite, ilmenite, and hematite) 

 complete the list of notable minerals in the rocks 

 of the area. Very small quantities of sulfates, 

 nitrates, carbonates, chlorides, and phosphates 

 occur, as well as a little gold, copper, silver, lead, 

 vanadium, nickel, chromium, barium, and tin — 

 free or in combinations. This means that the 

 12 or 15 elements most commonly found in the 

 rocks of the earth's crust (oxygen, silicon, alumi- 

 num, iron, calcium, magnesium, sodium, potassi- 

 um, titanium, hydrogen, phosphorus, carbon, 

 manganese, sulfur, and chlorine) occur in approxi- 

 mately that order in the Quiroga area. 



The feldspars in the rocks alter through weather- 

 ing to hydrous aluminum silicates (kaolinite or 

 claylike substances), calcite, sodium carbonate, 

 potassium carbonate, white mica, and quartz. 

 The olivine becomes serpentinized, as do the am- 

 philboles and pyroxenes, and iron oxides, calcite, 

 iddingsite, chlorite, epidote and quartz are 

 formed. In time such soluble minerals as calcite, 

 potassium hydroxide, and magnesite are removed 

 in solution, which leaves a final mixture of quartz, 

 claylike substances, and iron oxides. These are 

 the principal components of the soils in the area. 

 In other words, the gross body of the soils is com- 

 posed of a mixture of grains of quartz and par- 

 ticles of clay — commonly colored red or yellow by 

 iron oxides. Within this Si-0-H-Al-Fe matrix are 

 minor elements and compoimds altered or intro- 

 duced by water, air, plants, and animals. The 

 most important of these are organic carbon, 

 nitrogen, calcium carbonate (CaCos), sodium car- 

 bonate (Na2C03), sodium chloride (NaCl), sodium 

 and calcium sulfates (Na2S04) (CaSo4) in negligible 



