File:The Quarterly journal of the Geological Society of London (13960146853).jpg
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[edit]DescriptionThe Quarterly journal of the Geological Society of London (13960146853).jpg |
CRYSTALLINE ROCKS OF THE ALPS. 93 then (about 3^ miles below Zell) in situ, a cleavage-foliation being the dominant structure. Similar rock occurs a little way above Zell, where it is evidently greatly affected by pressure. In the upper part of the valley'around and above Maierhof, crystalline limestone, rather compacter in structure and greyer in colour than that south of the central watershed, is well developed, as, for instance, at the Calvarienberg below the village, and about a mile and a half above it, where the mass crosses the valley, and extends, according to Yon Hauer, far away eastwards and westwards. Its passage into calc- mica-schist and then into normal lead-coloured " Thonglimmerschiefer" is excellently seen in a knoll on the left bank of the valley, where the last two, like that already described, exhibit a distinct stratifica- tion-foliation *. The crystalline limestone often has a platy struc- ture, which strikes not far from E. and W., varying a few points on either side. In one place, above Haus, we see it almost in contact with a coarse gneiss in which a cleavage-foliation is very strongly marked, and of which the last outcrop is only about 10 feet from the last knoll of limestone. The cleavage-structures in the two rocks are not quite parallel, that in the gneiss being slightly S. of W., that in the limestone a little N". of it. At this point, we have either a granite intrusive in the limestone and crushed up with it, or a natural junction of an older and a newer series similarly crushed, or a faulted junction of the two rocks. As the limestone here is exactly in the same crystalline condition as it is in the three other localities where I examined it, I regard the first hypothesis as inadmissible. As regards the third, I saw nothing to suggest excep- tional sliding and shearing, but only direct crushing. Hence I do not think there has been an overthrust-fault, and so consider it on the whole more probable that limestone and other members of the " Thonglimmerschiefer " series were deposited upon an old surface of the gneiss. It is true there is no sign of a basal conglomerate, but this is not without precedent. Above Maierhof three mountain-glens pierce deep into the central chain of the Tyrol, giving the traveller another opportunity of exa- mining the " Central Gneiss " of Yon Hauer. We walked several miles up two of these, the Stillupthal and the Floitenthal, scruti- nizing the rocks in situ, the screes brought down by lateral torrents, and the erratics on the floor of the glen. In the Stillupthal we noted a porphyritic gneiss, with occasional darker bands of lenticular outline, also sundry gneisses and mica-schists, both light and dark, which bore some resemblance to gneisses of the Lepontine type. The first-named rocks are probably of igneous origin, the difference in appearance being due to differences in the effects of crushing. Some blocks also exhibited junctions which suggested an intrusion of the porphyritic rock into the " Lepontine " gneiss, anterior to the crushing, and the foldings in certain of the schists and gneisses are such as would be most naturally explained by supposing that a stratification- foliation existed when the cleavage-foliation was pro- For the microscopic structure of the rocks of this district, see Appendix, ■pp. 106-108. |
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Source | https://www.flickr.com/photos/biodivlibrary/13960146853 | ||
Author | Geological Society of London | ||
Full title InfoField | The Quarterly journal of the Geological Society of London. | ||
Page ID InfoField | 36939874 | ||
Item ID InfoField | 113696 (Find related Wikimedia Commons images) | ||
Title ID InfoField | 51125 | ||
Page numbers InfoField | Page 91 | ||
BHL Page URL InfoField | https://www.biodiversitylibrary.org/page/36939874 | ||
Page type InfoField | Text | ||
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Flickr posted date InfoField | 21 April 2014 | ||
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