The finny races of the sea and fresh waters amount to many thousand perhaps or more species; those yet recognised in a fossil state are aboutor one tenth; but, since a few years ago the number known was very inconsiderable, and new forms are continually presented to M. Agassiz, the master of this department of fossil zoology, there is reason to suppose that the proportion of recent and fossil numbers will steadily change. One reason of the comparative paucity of fossil fishes may be their enormous destruction for food; thus they perish in greater proportion than the other inhabitants of the sea.
Another way to think of texture is by the location of cooling.
Rocks which are glassy and vesicular are extrusive rocks. To learn about it, click here. The chart above may be a good one to memorize! Individual Rocks Below is a table of the rocks in the chart above.
Click the name of the rock to view an external link with more information. Click the pictures for larger views. Granite - a course-grained igneous rock composed of quartz, k-feldspar, and sometimes biotite.
Diorite is intermediate between granite and basalt Gabbro - a course-grained rock composed almost entirely of pyroxene and calcium-rich plagioclase Ca-plagioclasewith minor amounts of olivine Peridotite - a dense, course-grained rock consisting mostly of the minerals olivine and pyroxene Rhyolite - is an extrusive volcanic rock of felsic magma.
It is commonly made of quartz, feldspar, and plagioclase. It can be thought of as the extrusive equivalent to granite.
Had it cooled slower, it would have been granite. Andesite - an intermediate composition rock composed primarily of quartz and plagioclase, with accessory minerals present.
It is classified based on the most abundant phenocryst. For example, if olivine is the most abundant accessory mineral, it would be an olivine andesite. Basalt - This is the most common aphanitic rock.
It is very fine grained, usually dark colored, and originates with lava flows. It is composed primarily of calcium-rich plagioclase and pyroxene, with some amounts of olivine or amphibole. Pumice - a highly vesicular felsic igneous rock, usually light in color. It is formed as pyroclastic material is ejected into the air from a volcano Scoria - a highly vesicular igneous rock, usually dark brown, black, or red.
It is the mafic equivalent to pumice Obsidian - a naturally occurring glass produced by volcanoes. It is composed mostly of silica. Technically, it is not a mineral because it is not crystalline Tuff - a rock consisting of consolidated volcanic ash.
May contain shards of glass Tuesday - Reading and Research In order to properly understand igneous rocks, one must understand the environment where they solidified. There are two main environments that we need to examine. First, extrusive rock bodies are rocks which have extruded onto the surface of the earth and solidified.
Instead of discussing extrusive rocks here, we will cover it in the chapter on volcanoes. The second environment that we will consider here is intrusive rock bodies. We cannot observe this process, nor how long it takes. Many old geology texts had mere guesses at how long a large granite body took to cool, and they usually said millions of years.Chapter 5 Structural geologists think about deformation and flow as an inverse problem: from the current state back towards some (undeformed) initial state.
For example, the lower right photograph in the frontispiece for this chapter shows deformed oöids with elliptical shapes: ratios of long to short axes are about (Cloos, , ). The Products of Weathering and Erosion The main products of weathering and erosion are grains of quartz (because quartz is resistant to chemical weathering), clay minerals, iron oxide minerals, rock fragments, and a wide range of ions in solution.
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The new Governance bodies will takeover their functions from 1st August until the MedGeo Conference in The IMGA bylaws refer that “The period of service of Executive Committee shall be from the end of the MEDGEO at which it was elected to the end of the subsequent MEDGEO”.Due to the recent move of IMGA headquarters to Portugal, and the re-establishment of our By-Laws and.
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