Conglomeratic obsidian
Collection:
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Microscope
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Microscope

Fact sheet

Conglomeratic obsidian

Darwin wrote "A most remarkable conglomerate, consisting of mashed bits of porphyritic, jaspery, volcanic rocks, partly embedded in a ferrugineous base, but the whole mixed with patches & thin layers of pitchstone, passing into the nature of obsidian. I should think has certainly flowed as a lava with obsidian. Base of Central Hills. (The glassy rock is some igneous rock allied to the obsidian, partially decomposed, the obsidian greenish black: a concretionary pisolitic structure)"

Collected in July 1836.

Additional images
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Map
-7.933333, -14.366667
Description:
Ascension Island
Precision:
Poor
About this collection

This collection was a collaboration between The Open University and the Sedgwick Museum, created in 2009 to celebrate the bicentennial of the birth of Charles Darwin.

The Sedgwick Museum opened a new gallery, 'Darwin the Geologist', and created a museum-based virtual microscope to showcase rocks he collected during the Voyage of the Beagle. We enjoy this collection because Darwin did not always pick up a representative sample from the islands he visited; it's often the unusual rocks that caught his eye. Just explore the collection and you'll see what we mean. For those who have the opportunity, a visit to the museum is strongly recommended.

Sample details

Type
sedimentary
Category
conglomerate
Rock-forming mineral
glass
rock fragments
Category guide  
Category Guide
Title
Refers to any word or phrase that appears in the individual rock names. Names are generally descriptive; they allow users to search for broad terms like ‘granite’ as well as more specific names such as ‘breccia’. However, the adjacent descriptions of the specimens captures a wider range of general words and phrases and is a more powerful search tool.
Description
Refers to any word or phrase that appears anywhere in the descriptions of the specimens
Accessory minerals
Minerals that occur in very low abundance in a rock. They are usually not visible with the naked eye and contribute perhapssver, they often dominate the rare elements such as platinum group metals.
Rock-forming minerals
Minerals that make up the bulk of all rock samples and are also the ones used in rock classi?cation.
Timescale
Selecting one or more period, for example 'Jurassic'.
Theme
A term used to group together related samples that are not already gathered into a single Collection. For instance, there is a ‘SW England granites’ theme that includes such rock types as granite, hydrothermal breccia, skarn and vein samples.
Category
A general term used to label a rock sample. It is a useful way of grouping similar samples throughout a collection. Category names are often, but not exclusively, common rock names (e.g. granite, basalt, dolerite, gabbro, greisen, skarn, gneiss, amphibolite, limestone, sandstone).
Owner
The owner of the sample that appears in the collection. For example, NASA owns all the samples that appear in the Moon Rocks collection
We would like to thank the following for the use of this sample: