Abstract
IN a paper by me “On the Pre-Cambrian Rocks of Shropshire,” read before the Geological Society on the 11th inst., I call attention to certain granitoid and gneissic rocks in Primrose Hill, at the south-west end of the Wrekin. Associated with these metamorphic strata in such an irregular manner as to suggest an eruptive origin is a compact felspathic rock with minute quartz grains, which I at first presumed to be a quartz felsite. On submitting specimens to Prof. Bonney, F.R.S., for microscopic examination, he declared the rock to be clastic, and closely allied to the hälleflintas, which Dr. Hicks assigns to his Arvonian group. Certain observations recently made in south-west Shropshire, suggested tome a transition between the hälleflinta and the granitoid types, and, on communicating my suspicions to Prof. Bonney, he stated that a similar connection had been suggested by his microscopic examination. This morning I hammered over Primrose Hill foot by foot, and I have the satisfaction of announcing the fullest confirmation of our suspicions. In the same block, the compact hälleflinta is frequently mixed up with granitoidite and hornblendic gneiss. In some cases, the meta-morphism has taken place only near the surface, as if produced by atmospheric agencies; in others the crystallisation occurs in nests, while in others there is a gradual transition in mass from a compact to a granitic structure. This passage of hälleflinta into granite has obviously important theoretical applications.
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CALLAWAY, C. On the Origin of Certain Granitoid Rocks. Nature 20, 219 (1879). https://doi.org/10.1038/020219a0
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DOI: https://doi.org/10.1038/020219a0
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