The Quarterly journal of the Geological Society of London . nfusion. An irregularity of structure roughly suggestive of stratification(SchHeren (?) structure) is beautifully brought out by atmosphericweathering. AVhen exposed to water-action or embedded in thesoil, the blocks of foyaite are rounded like a normal intrusive exposed to the atmosphere on the hillsides, however, thereis a very general tendency to become fluted, as represented in the cuton the next page (tig. 2). This peculiar style of weathering does notappear to be confined to any ])articular type of structure or compo-
The Quarterly journal of the Geological Society of London . nfusion. An irregularity of structure roughly suggestive of stratification(SchHeren (?) structure) is beautifully brought out by atmosphericweathering. AVhen exposed to water-action or embedded in thesoil, the blocks of foyaite are rounded like a normal intrusive exposed to the atmosphere on the hillsides, however, thereis a very general tendency to become fluted, as represented in the cuton the next page (tig. 2). This peculiar style of weathering does notappear to be confined to any ])articular type of structure or compo-sition ; and on the worn or freshly-broken surface no appreciable 260 MR. O. A. PrRHY ON NEPHELTNE-ROCKS IN BRAZIL. difference in texture, to which the unequal action of the atmospherefan he attrihuterl, can be detected. The same feature is equallymarked in many of the rocks of the Serra do Itatiaia, but has notbeen observed in those of Poqos de Caldas and Cabo Frio, where, if itoccurs, it is not sufficiently prominent to have attracted attention. Fig. Fluted weathering of foyaite. From a photograph. Included frao;ments of other rocks, or of different types of thesame rock, are rare. The only one observed is a large anguLarpiece of augite-syenite, of medium grain and rich in magnetite,which affects a linear arrangement, giving a gneissoid aspect tothe rock*; but this lias not been found in independent of various kinds are extremely common. The mostabundant and characteristic are confined to the porphyritic type offoyaite, and present the aspect of crystals or groups of unusually perfect example, now preserved in the Xational Mu-seum, is represented in fig. 3 on the next page. These are evidentlyof the same nature as the smaller polyhedral inclusions in the truephonolite, which have been discussed by Graeff and Hussak f, and de-termined by the latter as pseudo-crystals in the form of leucite. Theaccompanying figure (fig. 4), from a photograph,
Size: 2157px × 1159px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1840, bookidquarte, booksubjectgeology
