GEOLOGY


formation and probably are of late Cretaceous, Paleocene, or early Eocene age. Cobbles of granodiorite, found by MacDonald in the gravels of Rio Cbagres and in conglomerate of the Bohio formation, presumably represent a group related to the Cretaceous(?) dioritic rocks.
 TERTIARY VOLCANIC AND INTRUSIVE ROCKS
 Though Tertiary lavas are found east of the canal and are widespread west of the canal, most of the Tertiary igneous rocks described by MacDonald and selected by him for chemical analysis were obtained from intrusive bodies.
 GRANULAR INTRUSIVE ROCKS
 Quartz diorite.-Cocovi Island, a small island in Panamn Bay west of the entrance to the canal, was found by MacDonald to be made up of quartz diorite porphyry. The rock is light gray but weathers almost white. It is markedly porphyritic, the phenocrysts, up to 6 millimeters in diameter, consisting of andesine, andesine-labradorite, some quartz, and a little orthoclase. Some of the feldspars are partly saussuritized. The ferromagnesian minerals are highly altered and for the most part unidentifiable; a few outlines of hornblende crystals were recognized. Though the finely crystalline groundmass is somewhat cloudy and altered, it seems to consist of plagioclase and shreds of ferronagnesian minerals. Magnetite, apatite, and chlorite are found in the rock. A chemical analysis of the porphyry is included in the table on page 55 (analysis 1).
 Augite quartz diorite forms Point Farfan, on the west side of the Pacific approach to the canal at the ferry terminus opposite La Boca. At Point Farfan, like on Cocovi Island, MacDonald obtained, by blasting, rock that proved to be considerably altered, although of fresh appearance. The quartz diorite at Point Farfan is gray and weathers light gray. In hand specimens it is slightly porphyritic and the groundmass is granular and almost medium grained. In thin sections andesine and somewhat altered augite are conspicuous. Quartz is present in small irregular masses, some of which appear to be secondary. Many small shreds of highly altered indeterminable ferromagnesian minerals were observed. Magnetite and apatite are accessory minerals and chlorite is the chief secondary mineral. The rock was analyzed and the results of the analysis are presented in the table on page 55 (analysis 2).
 Dacite.-The rock forming the Ancon Hill stock (between Ancon and Balboa), as well as Naos Island and Culebra Island in Panama Bay, was described as rhyolite by Howe (1908, p. 230-231) and MacDonald (1915, p. 28-29). In his manuscript MacDonald points out that although the rock has the appearance of rhyolite and some thin sections show as much quartz as shown


53


by many riyolites, the chemical analysis and additional microscopic examination show that it is dacite.
 The dacite at Ancon Hill is light gray acd weather to a light creamy color. As described by MacDonald, it has a fine-grained texture and some lathlike phenocrysts of plagioclase, the largest of which have faces measuring about 1 by 5 millimeters. In thin section the rock shows flow structure, particularly around the phienocrysts, which consist principally of andesine and some albite. Quartz in irregular grains, some augite, and a few small greatly altered needles of hornblende are present. The phenocrysts are widely scattered and grade in size into the coarser particles of the groundmass. Though the groundmass is somewhat cloudy and altered, it consists principally of perthitic aggregates of orthoclase and plagioclase and some quartz and feldspar intergrowths. Accessory minerals, in order of decreasing abundance, are magnetite, ilmenite, and apatite. A considerable amount of chlorite is present and scattered patches of an unidentified light yellowish secondary mineral show in the groundmass. (See analysis 3, p. 55.) W. S. Burbank suggests that MacDonald's description and the chemical analysis indicate that the rock is considerably altered, principally by processes allied to albitization. During the construction of the canal a quarry on the west face of Ancon Hill, at a locality now known as Quarry Heights, furnished great quantities of this dacite for use in concrete in the construction of Mirafiores and Pedro Miguel Locks.
 The stocks of porphyry between the canal and Madden basin, northeast of Gaillard Cut, include dacite porphyry, according to geologists of the Geological Section of the Special Engineering Division. The porphyry intruding the Gatuncillo formation in the Rio Casaya area (locality 38), for example, is dacite porphyry. The borders of this stock and the intruded rocks are slightly mineralized and socee mining operations were carried on many years ago, as described on p. 59. MacDonald thought some of the rock in this area probably is granodiorite, but he found nothing suitable for microscopic examination.
 Diorite.-A minor faces of the quartz diorite at Point Farfan is described by MacDonald as quartz-bearing gabbro. W. S. Burbank, however, points out that MacDonald's description of the mineralogical composition and the chemical analysis indicate a classification near diorite. The rock is dark gray, medium-grained, and equigranular. The principal minerals, arranged in approximate order of relative abundance, are andesine, augite, and oligoclase. Quartz in small irregular patches is a minor constituent, which W. S. Burbank suggests may be secondary. Accessory magnetite, apatite, and ilmenite are present. Chlorite is found ice the rock and some of the feldspars show slight saus-