══════════════╤══════════════════════════════════╤════════════════ │ AVERAGE IGNEOUS ROCK │ ├——————————————————┬———————————————┤ AVERAGE SHALE │ (Clark) │ (Washington) │ ——————————————┼——————————————————┼———————————————┼———————————————— │ │ │ SiO_2 │ 61.25 │ 61.69 │ 63.34 Al_{2}O_3 │ 15.81 │ 15.94 │ 16.56 Fe_{2}O_3 │ 2.70} 6.31 │ 1.88} 4.53 │ 4.41} 7.89 FeO │ 3.61} │ 2.65} │ 3.48} MgO │ 4.47 │ 4.90 │ 3.54 CaO │ 5.03 │ 5.02 │ 3.33 Na_{2}O │ 3.64 │ 4.09 │ 1.29 K_{2}O │ 2.87 │ 3.35 │ 3.52 TiO_2 │ .62 │ .48 │ .53 │ —————— │ —————— │ —————— │ 100.00 │ 100.00 │ 100.00 ══════════════╧══════════════════╧═══════════════╧════════════════ This close resemblance is probably of deep significance, for the reason that shales and slates are structurally the weakest of all rocks and for the further reason that they rather generally directly underlie the carbonate rocks, which are by contrast the strongest (see _ante_, p. 37). For these reasons shales and slates are the only rocks which are likely to be fused by relief from load through the formation of anticlinal arches within the earth’s zone of flow. If this view is well founded, lavas and other igneous rocks are in large part fused argillaceous sediments formed in connection with the process of folding, or are refused rocks of igneous origin and similar composition. =Character profiles.=—The character profiles of features connected in their origin with volcanoes are particularly easy to recognize, and in a few cases in which they might be confused with others of a different origin, an examination of the materials of the features should lead to a definitive judgment. The lava plains which result from massive outflows of basalt might perhaps strictly be regarded as lack of feature, so great may be their continuous extent. Wherever definite vents exist, a broad flat dome is the usual result of the extravasation of a basaltic lava. The puys of France and many of the Kuppen of Germany, being formed from less fluid lava, have afforded profiles with relatively small radius of curvature.