Year: 1906
Type: book
Abstract: half, and Sabatini of the eastern.For the present, the question must be left an open one, and this is the more advisable here, since, whatever be the facts as to the origin of the topographic features and structure, they have little-bearing on the predom- inantly petrological questions treated of in the present paper.Ciminian District.The Ciminian District immediately adjoins the preceding one on the north, and is bounded on the south by the Sabatinian District.Its area is approximately 900 square kilometers.It is composed of two distinct volcanoes, that of Cimino on the north and of Vico on the south, the former being the earlier.The remains of the Cimino Volcano are made up of several hills, including Monti Pallanzana, Vitorchiano, Cigliano, and Soriano, surrounding on the west, north, and east the main mass of Monte Cimino proper, whose highest point has an altitude of 1,035 meters above sea-level.This complex is pretty clearly the remnant of a single volcano, though the original structure is largely hidden by the extensive erosion which it has undergone and which has been much aided by the tuff-like character of most of its rocks.Immediately to the south, or rather south-southwest, of Monte Cimino, whose southern flanks are partly covered by its ejectamenta, is the better-preserved Vico Vol- cano.Vico is a fairly well-preserved volcanic cone of the normal type of strato-volcano, the sides sloping up regularly (much more so than in the Vulsinian District) to the circular crest, inside which is the small, shallow Lake Vico.North of this lake is the site of the last eruption of the volcano, Monte Venere, a small hill, rising 325 meters above the lake-level, and composed of lava flows with some tuffs and lapilli.The lake surface is 507 meters above sea-level, and the steep inner wall of Vico rises from loo meters above this on the south, where it is the lowest, to 456 meters at Monte Fogliano on the west.Though the almost perfect circularity of the rim is marred by the projection of this last summit, there seems to be no doubt that the hollow is a true crater and the volcano a simple one, with few parasitic cones.The feature of radial valleys eroded in the loose surrounding tuffs is well marked in the topography of this volcano.Of the towns which may be mentioned, the city of Viterbo lies in the angle between the two volcanoes, west of Soriano and north of Vico.About the Cimino Volcano we find the villages of Soriano on the east, Vitorchiano a short distance to the north, and Bagnaia on the northwest, while around Vico are San Martino near the northwest crater rim, Vetralla on the west, Capranica on the southern edge of the area, Ronciglione near the southeast shore of Lake Vico, and Caprarola and Canepina to the east and northeast. Sabatinian District.This lies immediately to the south of the Vico Volcano, with whose tuffs its own intermingle.The area covered by volcanic rocks is estimated by Moderni at 1,369 square kilometers, so that in extent it is intermediate between the Vulsinian and the Ciminian districts.It resembles the district first described, inasmuch as a sheet of water, Lake Bracciano, forms the topographic center.This lake, with an area of 57.5 kilometers and a depth of 160 meters, has its surface 164 meters above sea- level.It is approximately circular in shape, with two or three small bays, and is surrounded on the west and north by a girdle of hills, which attain their highest altitude above sea-level (602 meters) at Monte di Rocca Romana on the north.Im- mediately to the east is the small Lago di Martignano, and the basins of two others are represented in the Stracciacappa Marsh and the Valle di Baccano, all three being distinctly elliptical or circular and surrounded by rings of hills or curving ridges.From the inner wall surrounding Lake Bracciano the land slopes down to the west, north and east rather regularly, though it is broken on the west by the Tertiary volcanic masses of Monte Calvario and Monte San Vito.On the south the hills along the lake shore are lower, and the slope southward is very gentle.The general topographic resemblance to the Vulsinian District is very marked, though the features are on a somewhat smaller scale.Moderni, who has studied this district very carefully, argues for it a structure analogous to that which he assigns to the Vulsinian.He regards Lake Bracciano as being, not a crater lake, but as a lake basin formed by the ejections of the surrounding volcanic vents, of which he enumerates and describes eight main ones, with their parasitic cones.This view is combated by Sabatini, so that, for the present, the question of the structure of the complex and the origin of the lake must be regarded as unsettled.Of the towns in the district the most important is Bracciano, on the crest of the western lake-rim.Smaller ones are Manziana and Oriolo on the northwest, Trevignano on the northern shore, near a small semicircular bay, and Anguillara on the southeast shore near the outlet of the lake.Latian District.The Alban Hills, which make up this district, are situated to the southeast of Rome, as is well known, and some 50 kilometers southeast of Lake Bracciano.The area covered by the volcanic complex is about 580 square kilometers.The complex consists of a circle of hills and summits, varying in altitude from 550 to 939 meters, and with a diameter of about 15 kilometers.Up to this the sur- face rises gently from the north, east, and south, while on the west and southwest the regularity of this somma is broken by the lakes Albano and Nemi and the Valle Ariccia, the sites of late flanking eruptions.Inside the large circle there rises a fairly well-defined cone, attaining a height of 956 meters and with the crater known as Campo d'Annibale at the top.Monte Cavo forms the most prominent and * Washington, Prof. Paper U. S. Geol.Surv.No. 14, 1903, p. 44.t Iddings, Prof. Paper U. S. Geol.Surv.No. 18, 1903, p. 72 and Plate II.in percentages, and that molecularly the former may surpass the latter.The exact classifkatory position of this type is clear from the ratios.As regards class, order, and rang, it falls well within the borders of persalane, canadare, and nordmarkase ; but the ratio of the alkalis shows that the rock is transitional between the sodipo- tassic subrang phlegrose, in which it actually falls, and the dosodic subrang nord- markose.This type must therefore be called a nordmarkose-phlegrose.Chemical Composition 0} Ischial Nordmarkose-phlegrose [Augite-trachyte}.The wollastonite which was left over was placed in the aegirite-augite molecule though it is a trifle high.There not being enough SiO 2 left to form albite from all the Na 2 O present, nephelite was calculated by the same set of equations used to cal- culate the norm, exactly as was done in the preceding mode.The average alkali- feldspar, then, will be Or 97 Ab 4 8, or almost exactly O^Abj.Comparison with the measured mode shows the same relations as in the case of the Cuma rock, though here it is considerably greater, and is also to be attributed to overlapping.The calculated and measured amounts of haiiyne are very concordant, which in both cases is to be attributed to the rather fair size of the crystals, equaling or excelling the thickness of the section, and their equant shape and consequent small liability to overlapping.* This description applies to the lava form of the type, as the breccia form was not readily measured, and its brecciated condition also does not make a formal description necessary.Augite.About 25 per cent, 0.05 to 0.50 mm., subhedral to anhedral, partly in prismatic and equant, partly irregular and interstitial; pale gray, few inclusions.Olivine.About 7 per cent, 0.05 to o. 10 mm., anhedral, equant.Melilite.About 12 per cent, anhedral, interstitial patches, sometimes poikilitic about the smaller crystals, areas up to 2 mm. in diameter, "pflock" structure common, pale yellowish.Magnetite.About 4 per cent, 0.02 to 0.05 mm., anhedral, equant.Apatite.About i per cent, 0.03 to 0.08 mm., subhedral, prismatic.Chemical composition and norm as on p. 139.Type specimen from Capo di Bove, Latian District, Correlation of Types.
DOI: 10.5962/t.174873
Author: Henry Stephens Washington
Language: English
Subfield: Geophysics
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: gold
OpenAlex ID: W637940912