V. Some important points suggested by the previous discussion. A. Successive outflows of Oascade lava.-Mountain-making, w hetller by the up-swelling of sediments or the out-squeezing of liquid, is probably always a slow process. The latter mode, however, viz: the up. . builrling by out. . squeezed matters is probably more paroxysmal than the former. 'Ehe Cascade Range was probably built up paroxysmally by many successive outflows, occupying a very long period of time. I believe it possible to detect these successive outflows-to detect in its exogenous structure the lines of mountain-growth. It is well known that columnar basalt, when in thiek masses, is often divided into horizontal layers by planes which inter. . rupt the continuity of the vertical columns. Magnificent exam pIes of this structure are found in the basalts of the Cascade Range. On the nearly perpendicular cliffs of Columbia Ri ver, ten or twelve horizontal layers may be distinctly counted (figs. 2 and 3), and doubtless many others exist, but their out. . cl"Opping edges are covered with the products of disintegration. On the perpendicular cliffs of the Des Chutes River (fig. 1), at least twenty layers can be clearly made out, and about ten more with less certainty, on one section. Still higher up on the gentler slope are seen many others, thOllgh their out-cropping edges are mostly covered up by debris and soil. That these
JN the series of rocks representing the history of the earth there occur at different horizons unconformities. In most cases these are not found at the same horizon in different places; but there are a few which seem to be very general. Associated with these unconformities, as is well known, there is nearly always a marked change in the fossil species. The greatness of this change is invariably in direct proportion to the generality of the unconformity. These general unconformities attended with very great changes in organic forms are the natural boundaries of the great divisions of time, and the less general unconformities attended with less sweeping change of organic forms, of the subdivisions of time.The earlier geologists, under the influence of the then domninant idea of frequent supernatural interference with the course of nature, imagined that these unconformities marked the times of instantaneous cataclysm which disturbed the rocks and destroyed all living things, sometimes locally, sometimes generally, and that these exterminations were followed by re-creations of other and wholly different species at the beginning of the subsequent period of tranquillity. Now, however, we believe that no such instantaneous general exterminations and re-creations ever occurred. We know that unconformity simply indicates eroded land-surface, and therefore marks a period of time during which the observed place was land and received no sediment; that two series of rocks unconformable to each other denote two periods of comparative quiet, during which the observed place was sea-bottom, receiving sediment steadily, separated by a period of oscillation producing increase and decrease of land, during which the observed place was raised into land-surface, with or without crumpling of the strata, deeply eroded, and then sunk again below sea level to receive the second series of strata. The length of the two periods of repose is roughly measured by the thickness of the two conformable series. The length of the period of commotion is roughly measured by the amount of erosion at the line of unconformity.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.