Porphyry Intrusions Research Articles

Stoping and assimilation in a granodiorite stock at Jamestown, Colorado

At Jamestown, Colorado, a granodiorite stock of late Cretaceous or early Eocene age has intruded into pre‐Cambrian schists and granites. Though there is some evidence that the magma intruded partly by pushing aside the enclosing rocks, there is also considerable evidence that stoping and assimilation played an important part in the emplacement of the stock. Schist breccia at the border and several small xenoliths scattered in the stock seem to represent various stages in the process of assimilation.The granodiorite stock is in the Jamestown Mining District, 33 miles northwest of Denver, and at the extreme northeast end of the Front Range mineral belt. It is one of the earliest of a series of porphyry intrusions that show a close genetic relationship to the ore deposits of the District.

Petrology, Structure, and Relation to Tectonics of Porphyry Intrusions in the Beartooth Mountains, Montana

Many of the porphyry intrusions of the northeastern Beartooth Mountains are in close proximity to the Beartooth thrust fault which is thought to have been developed during the Laramide orogeny. Detailed studies of the porphyry bodies show that they were intruded before the thrusting took place and orogenically have no relation to the compressional episodes expressed by the thrusting. The shape, structural and age relations, distribution, and mineral composition of the different porphyry intrusives are described.

The Flora of Warwickshire

THE interest to outsiders in the plants of Warwickshire lies in the fact that we have here a typical English Midland county, the botany of which is not in any way affected by proximity to the sea or high mountains. Although it is the central county of England, and forms the watershed of the Severn, Trent, and Thames, no portion of its surface rises above 855 feet. Its area is 885 square miles, or 566,458 acres, and it contains 4 hundreds, 2 cities, I county town, 10 market towns, and 209 parishes. In 1888 the crops of corn, beans, and peas occupied 106,000 acres; green crops, 38,602 acres; permanent pasture land, 312,000 acres; fallow, 8161 acres; and woodland, 16,650 acres. The soils are fertile, but varied, comprising nearly all kinds but those containing chalk and flints. All the southern and south-eastern part of the county is occupied by a strong clay resting on limestone. A soil of similar kind occupies the north-west. Over a large portion of the county, extending from Warwick to its western boundary, are strong clay loams resting on marl and limestone. Westward and south-westward of Warwick there is a strong clay over limestone. About Rugby and in the valleys of the Blythe and Tame are light sandy soils mixed with gravel. The remaining extensive portions of the county consist of a red sandy loam and a red clay loam, resting on freestone or limestone, and sometimes on gravel. The extent of unin-closed land is very inconsiderable, the only extensive commons being those of Sutton Coldfield and Yanin-gale. The subjacent sedimentary rocks begin with the Cambrian and end with the Inferior Oolite, with a little volcanic tuff with intrusions of diabase and quartz porphyry in the north-east, near Atherstone. The Flora of Warwickshire. “The Flowering Plants, Ferns, Mosses, and Lichens,” by James E. Bagnall., Associate of the Linnean Society.” “The Fungi,” by W. B. Grove, M.A., and J. E. Bagnall. 518 + 34 pages, with a Map. (London: Gurney and Jackson. Birmingham: Cornish Brothers, 1890.)

Notes on the Geology of New South Wales

Abstract T he author alludes to the regular succession downwards of—1. Coal-bearing beds; 2. Rocks with “Lower Carboniferous” or “Devonian?” fossils, and with Coal-plants and Coal; 3. Metamorphosed grits and fine conglomerates, derived from granitic and porphyritic rocks, occasionally fossiliferous, and sometimes enclosing calcareous beds. This porphyritic grit and conglomerate the author regards as being referable to Von Humboldt's Lozero of Mexico, which underlies the Grès houiller of Quanaxato. On the western flanks of the southern part of the New England County there are conglomerates, referable to the base of the Carboniferous rocks of the flanks of the Liverpool Range, which are associated with Devonian? fossils and altered grits and shales full of Lepidodendron . 4. The great north and south schistose formations, in which the author has found fossils which he assigns with some doubt to the Silurian epoch. The schists have been broken through, after being metamorphosed, and before the deposition of the superjacent and unconformable grits, by intrusive porphyry. Mr. Clarke also refers to the existence of fossil fish [of which he has sent “rubbings”] in the upper rocks of Sydney. They have some resemblance to the Permian fish; and, if they be true indication of the Permian character of the beds, “ we are able to show,” says the author, “an unbroken series from the Permian rocks to the representatives of the Old Red (?).”