PR01'\ T. G. BONNEY ON BEADGATE-PAKK SYENITE. 101 



i). Note on a Contact-structuke iu the ISyenite of Bkadgate Park. 

 By T. G. BoNNEY, D.Sc, LL.D., E.R.S., V.P.G.S., Professor of 

 Geology in University College, London, and Eellow of St. John's 

 College, Cambridge. (Eead January 7, 1891.) 



Built into a wall at the ruins, Bradgate, is a block which exhibits 

 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 compara- 

 tively low temperature in the intrusive rock ? Microscopic exami- 

 nation 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 Porest. If a slice were cut exclusively 

 from this part it might be passed over without any suspicion of its 

 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 diff'erences. 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 has at its margin an ill-defined zonei, 

 about "06" wide, consisting of a microgranular matrix, in which are 

 many small fragments (apparently of felspar and possibly of quartz), 

 very like one of the more gritty bands which may be seen in the slate 

 on Target Hill. To this succeed crystals of felspar and grains of 

 quartz, say about "04" in diameter, of which the former sometimes 

 exhibit regular crystalline outlines, sometimes seem to be fragmental 

 and scattered in a matrix, described below. These increase so 

 rapidly in number that at least half the slide is occupied by them. 

 The minor interstices are occupied by viridite, but the larger exhibit 

 the " speckled " devitrified structure so common in lava-fragments 

 from the Porest agglomerates and iu the Sharpley rock. In one 

 or two, however, somewhat ill-defined, lath-like crystallites of felspar 

 occur, such as may be seen in many trachytic rocks, and these some- 

 times exhibit an approach to a spherulitic grouping. The figure on 

 the next page (1) is a careful drawing from the most conspicuous 

 instance, and the diagram (fig. 2) gives an idea of the relative pro- 

 portion of matrix and crystals. 



A comparison of the structure described above with that generally 

 presented by the " syenite *' in other parts of the Porest * seems to 



* See Hill and Eonney, Quart. Journ. Geol. Soc. vol. xxxiv. (1878) p. 211, and 

 Mr. Teall's fuller and improved description, ' British Petrography,' p. 270, &c. 



