Note on a Contact-Structure in the Syenite of Bradgate Park

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Abstract
B uilt into a wall at the ruins, Bradgate, is a block which exhibit a clear junction between the “syenite” and a pale green argillite Microscopic examination of a fragment which we contrived to detach throws light on an apparent anomaly and offers some suggestions of a wider bearing. The argillite, though in contact with a rock apparently rather coarsely crystalline, is only converted into a natural porcelain; it is “baked” rather than “metamorphosed.” Is this due to the refractory nature of the materials or to a comparatively low temperature in the intrusive rock ? Microscopic examination shows that within a quarter of an inch of the actual junction the argillite does not materially differ from one of the “flinty slates” common in the Forest. If a slice were cut exclusively from this part it might be passed over without any suspicion of it proximity to a contact-surface. The syenite, macroscopically, is slightly finer in grain, and less definitely mottled with dull green than the normal rock, but on microscopic examination it exhibits some important differences. The line of junction of the two rocks is slightly wavy; for about ·02″ to ·04″ the argillite is darkened and one or two tiny roundish patches occur, occupied by viridite and chalcedony (?), which may possibly be minute cavities subsequently filled. The intrusive rock lias at its margin an ill-defined zone, about ·06″ wide, consisting of a microgranular matrix, in which an many small fragments (apparently of felspar and possibly of quartz) very like

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