PETROLOGY. 213 



Hoppe-Seyler, F. Ueber die Bildung von Dolomit. [Formation of 

 Dolomite. J Zeitsch. deutsch. rjeol. Ges. Heft iii. pp. 495-530; plate. 



The author's experiments lead him to conclude that dolomite cannot 

 be artificially produced at ordinary temperatures. He maintains that 

 the magnesia was not derived from basaltic or other eruptive rocks, but 

 from the sea. Submarine volcanic eruptions have raised the temperature 

 sufficiently to admit of the formation of dolomite ; but the magnesia 

 required for the dolomitization of large masses of limestone was 

 obtained from sea- water. F. W. 11. 



Hull, Prof. Edward. On the Microscopic Structure of a frngment 

 of " Baked " or Indurated Slate, from the Lower Silurian Rocks, 

 Claremont Hill, near Dundalk. Journ. 11. Geol. Soc. Ireland, 

 ser. 2, vol. iv. part 2, pp. 85-88 ; plate. 

 The specimen was from a dark grey, compact, hard splintery rock. 

 Trap rocks abound in the neighbourhood ; and the induration is attri- 

 buted to a high temperature. Under the microscope the rock is seen 

 to consist of a colourless glass with rounded and subangular grains of 

 silica and grains of magnetite. An analysis by Rev. S. Haughton 

 shows the presence of all substances needed to produce a glass, alkalies 

 and lime being abundant. Thus the clay has been vitrified, the silica 

 grains remaining almost unaltered. The metamorphism had begun to 

 develop felspar, some little prisms of which are seen. An Appendix 

 gives a note on the proportion of Magnetic Iron-ore in the specimen, by 

 Mr. E. T. Hardman ; and a letter from Mr. G. J. Stoney on the 

 probable temperature at which the partial fusion took place. E. T. H. 



. Constituents of Leinster Granite. Quart. Joinm. Micr. Sci, 



vol. XV. p. 102. (Dublin Micr. Club, 23 July, 1874.) 

 Contains an orthoclase and a plagioclase felspar, believed to be albite. 



. Composition of Vesuvian Lava. Quart. Journ. Ali-cr. Sci. 



vol. XV. p. 330. 

 Notice of sections of lava exhibited at the Dublin Micr. Club. 



Hunt, Dr. T. S. On the decayed Eocks of Hoosac Mountain. 

 Trans. Amer. Inst. Min. Eng. vol. iii. pp. 187, 188. 



The rockin the tunnel through the mountain consists of mica-schist 

 and micaceous gneiss, including in its W. half a mass of harder fel- 

 spathic and quartzose strata, mostly dipping E. Complete decomposition 

 W. was observed at the base of the mountain, the felspar being con- 

 verted into white clay, and the mica much softened and disintegrated. . 

 The decomposition took place before the Drift period. K. B. N. 



. Prof. J. D. Dana on the Alteration of Rocks. Proc. Boston 



Nat. Hist. Soc. vol. xviii. pp. 108-111. 



Kalkowsky, Dr. Ernst. Mikroskopische Untersuchung dcs Glim- 

 mertrapps von Metzdorf. [Mica-trap of Metzdorf.] N. Jahrh, 

 Heft V. pp. 488-503. 



A rock called mica-trap, occurring near ;^^etzdorf in the gneiss dis- 

 trict of the Saxon Erzgebirge, is described under 3 heads: — 1. Its 



