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IV.—On Brecciated Concretions

Published online by Cambridge University Press:  01 May 2009

John Ruskin
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Extract

The states of semi-crystalline silica are so various, and so connected in their variety, that the best recent authorities have been content to group them all with quartz, giving to each only a few words of special notice; even the important chapters of Bischof describe rather their states of decomposition and transition than the minerals themselves. Nevertheless, as central types, five conditions of silica are definable, structurally, if not chemically, distinct; and forming true species: and in entering on any detailed examination of agatescent arrangements, it is quite necessary to define with precision these typical substances, and their relation to crystalline quartz.

Type
Original Articles
Information
Geological Magazine , Volume 5 , Issue 43 , January 1868 , pp. 12 - 18
Copyright
Copyright © Cambridge University Press 1868

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References

page 15 note 1 In my woodcut diagrams I shall employ no fine execution; they will be merely illustrative, not imitative,—diagrams, not drawings. In the plates, on the contrary, with Mr. Allen's good help, I shall do the best I can.

page 15 note 2 Or green earth? I cannot find any good account of the green substance which plays so important a part in the exterior coats of agates, and Iceland chalcedonies.

page 16 note 1 I use this word gravity in some doubt; not being quite sure that the straight beds are always horizontal, or always inferior to the rest deposited at the same time. I have one specimen in which, according to all analogies of structure, it would appear that the vacant space is under the level floor, between it and reniform chalcedony; and sometimes these floors cross pillars of stalaetite like tiers of scaffolding.