. Bulletin . avertical section thiough them would look somewhat like a vertical sec-tion through the upper part of the Bas Obispo, and a part of the over-lying Las Cascadas agglomerate, with some higher-bedded tuffs in-cluded. The evidence indicates that these meta-breccia masses werepushed upward as somewhat metamorphosed and toughened caps ontop of basalt plugs or cores. On cooling, shrinkage and outlets forvarious dikes and apophyses caused a gradual settling back of theseplugs to about their present condition. These breccia masses haveacted as strengthening pUlars to buttress some of the s
. Bulletin . avertical section thiough them would look somewhat like a vertical sec-tion through the upper part of the Bas Obispo, and a part of the over-lying Las Cascadas agglomerate, with some higher-bedded tuffs in-cluded. The evidence indicates that these meta-breccia masses werepushed upward as somewhat metamorphosed and toughened caps ontop of basalt plugs or cores. On cooling, shrinkage and outlets forvarious dikes and apophyses caused a gradual settling back of theseplugs to about their present condition. These breccia masses haveacted as strengthening pUlars to buttress some of the sliding areas inCulebra Cut. This function is discussed in a subsequent considera-tion of shdes. LAVA-MUD FLOWS. In addition to the igneous rocks enumerated, there are severalcooled and hardened mud-lava flows which are found in the LasCascadas agglomerate. Many of these local masses show columnarstructure (see PL XXVIII) and are rather hard, but on exposure tothe air for a few years they crumble GENERAL GEOLOGY OF CANAL ZONE. 31 STABILITY OF THE ISTHMUS. A brief reconnaissance of the geology of western Panama was madepossible by a cooperative arrangement between the Isthmian CanalCommission, the Smithsonian Institution, and the United StatesGeological Survey. The information thus gathered, coupled withthat obtained from a study of the Canal Zone geology, leads to theconclusion that the isthmian land first appeared as an archipelago ofislands in a shallow sea. During the Oligocene period, when the Cu-curacha formation was laid down, there seems to have been estabhshed,possibly for the first time, a land connection between the two conti-nents. At least there is, as yet, no positive geologic evidence of anearlier connection. Before the close of the Oligocene, the sea againswept over the submerged land, leaving only a few islands above thesurface of the water. It appears that the Mocene began with anemergence, and that until its close the isthmian barrier once moreef
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