The white layers which occur frequently as infillings of joint-cracks in English coal are miniature mineral-veins. Though averaging less than a millimetre in thickness in ordinary specimens, these veins are in many cases very numerous, and their presence constitutes an important feature in the composition of the coal. As a rule, the dominant mineral is ankerite, which may be normal in composition (i. e. correspond to the formula 2 CaCO₃. Fe(Mn)CO₃. MgCO₃), or may contain a small excess of calcium carbonate, apparently in the form of an isomorphous growth. Calcite also occurs, and is sometimes the dominant mineral. The other associated minerals are iron-pyrites, barytes, zinc-blende, and galena.