Tepetates classification has been based on the type of cementing substance. When the cementing substance is mainly silica (Si[O.sub.2]), the tepetates are denoted "duripan"; when is calcium carbonate (CaC[O.sub.3]), the tepetates are denoted "petrocalcic" type; when is calcium sulfate (CaS[O.sub.4]), they are denoted "petrogypsic" type; when are salts, they are denoted "petrosalt" type; when is iron, they are denoted "petroferric" type and when is clay, they are denoted "fragipan" type.
There are also reports focused on the cementing substance identification, the use of soil, and its ecological significance [6, 7].
The liberated compounds are lixiviated and deposited and then, they act as cementing substance (free silica, calcium carbonate) in the soil.
The strongest, most indurated tepetate appears to occur when disseminated carbonates combine with silica as cementing substances .
Selenium: As a constituent of the enzyme glutathione peroxidase, selenium contributes to the protection and maintenance of physiological function of the lipid rich intercellular cementing substance. The recommended level of selenium in dairy diets is 0.3 mg/kg DM (NRC, 2001).
Vitamin E plays an important role in maintaining the integrity of keratinized tissues as the intercellular cementing substance is composed of lipid rich material (Mulling et al., 1999).
The intercellular cementing substances, composed of glycoproteins and complex lipids, are important for epidermal adhesion (Tomlinson et al., 2004).