GEOLOGY AND PALEONTOLOGY OF CANAL ZONE


suritization. W. S. Burbank points out that the soda content of the analysis of this rock on page 55 (analysis 4) is slightly high for normal diorite and suggests that the alteration recorded by MacDonald may indicate weak albitization.
 Andesite.-Andesite porphyry forms some of the stocks in the area between the canal and Madden basin. A small stock of hornblende andesite, characterized by conspicuous needles of hornblende, is being quarried along the road between Miraflores Lake and the Transisthmian Highway.
 DIKE ROCKS
 Andesite.-Andesitic dike rocks that cut the Las Cascadas agglomerate are mentioned by MacDonald.
 Basalt.-Dikes and small irregular intrusive bodies of almost black rocks, all grouped as basalt, are widely distributed southwestward from the southeastern part of the Gatun Lake area and the southern part of Madden basin. The irregular intrusive bodies form hills; in fact, most of the high hills in the southeastern part of the Canal Zone are formed by intrusive or extrusive basalt.
 Basalt obtained from dikes at 10 localities, for the most part in Gaillard Cut, was examined by MacDonald. These rocks are very dark and fine-grained. Labradorite, andesine, and augite are the principal constituents among the larger crystals. Some of the rocks also contain enstatite and a little biotite. The groundmass is made up of laths of plagioclase and grains of augite, but generally includes a little glassy material. Magnetite and ilmenite are the chief accessory minerals. Some chlorite and a few patches of serpentine -possibly an alteration product of olivine-are present.
 Basalt from a dike on the Panama Railroad 3 kilometers northwest of Monte Lirio was selected for chemical analysis. (See table, p. 55, analysis 5.) As described by MacDonald, the rock is of coarser texture than the usual basalt in the Canal Zone and its feldspars are more calcic. Hand specimens show crystal faces of pyroxene that shine with a resinous luster and have a maximum diameter of 4 millimeters. Under the microscope the largest phenocrysts are seen to be augite. The feldspar crystals, slightly in excess of the ferromagnesian minerals, are for the most part labradorite and some of them are zoned. A little andesine is present. Augite occurs in granular aggregates as well as in phenocrysts. Magnetite, apatite, some ilmenite, and a few small grains of zircon are present. Secondary minerals consist of numerous patches of iron oxide(?), some serpentine that may be an alteration product of olivine, and a few small patches of chlorite. Some of the feldspar crystals are partly saussuritized and in zoned crystals the alteration is zonally selective. This


rock was quarried for use in facing the water-level part of Gatun Dam.
 VOLCANIC ROCKS AND TUFF
 Dacite.-Dacitic glassy lava from the Las Cascadas agglomerate is included in the rocks selected for chemical analysis (see analysis 6, p. 55). According to MacDonald's description, the glassy lava forms the matrix of thin flow breccia. The brecciated fragments enclosed in the glassy matrix consist chiefly of pyroclastic rocks of the Las Cascadas. Some of the glassy lava contains elongated gas cavities drawn out in the direction of flow.
 The hard dacitic tuff in the Cucaracha formation, mentioned in the description of that formation, was also analyzed (analysis 7, p. 55). Thin sections of the tuff were examined by W. S. Burbank and R. L. Smith, who found it to be a welded tuff. The glassy base consists of compressed glass shards, for the most part partly or entirely altered to clay. Some of the plagioclase (andesine-labradorite) crystals are euhedral; others are fragmental. Decomposition products of a few unidentifiable ferromagnesian minerals are recognizable and the rock has a few veinlets of calcite. Fragments of finely crystalline lavas of varying composition are scattered through the tuff. The dark little lentils, conspicuous in hand specimens, consist of saponite (a clay mineral of the montmorillonite group), evidently an alteration product of compressed pumice lapillae.
 Andesite. -MacDonald mentioned andesitic flow breccias and dark coarse-grained andesitic flows in the Las Cascadas agglomerate.
 Basalt.-Some of the basalt in the southern part of the Canal Zone consists of remnants of flows and the undifferentiated volcanic rocks in the southwestern part of the map area include much basaltic lava.
 A flow remnant capping Gold Hill, which forms the continental divide on the east side of Gaillard Cut, is described by MacDonald as dark fine-grained basalt. The larger crystals consist of feldspar, mostly labradorite, and augite. Augite also occurs as grains and irregular aggregates. The groundmass is distinctly crystalline, though very fine-grained. It has about the same composition as the larger crystals. Small grains and irregular aggregates of magnetite, some apatite, and a little ilmenite are scattered through the rock. Epidote in light and dark yellow irregular patches fills cracks in broken feldspars and occurs as cloudy masses in the interior of some feldspar crystals. The thin sections show no olivine, but a light yellow secondary mineral may represent altered remnants of olivine. A chemical analysis (No. 8) of this rock is included in the table on page 55.


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