324 Revieivs — Brief Notices 



Nat. Museum, 1908, toL xxxv, pp. 133-54, with seven plates). 

 After some general remarks on geodic cavities "which are common in 

 all kinds of rock, the author refers more particular!}- to those in lime- 

 stones and shales, and to the Niagara limestone of Lockport, -which is 

 rich in examples. There the cavities in many cases are due to the 

 removal by solution of fossils, or they may be fractures in the strata 

 enlarged by the expanding force of crystallization ; the mineral matter 

 lining the cavities is usually dolomite, gypsum, and calcite. Frequently 

 fossils themselves, such as Erachiopoda, furnish the cavities and are 

 lined with crystalline material. The most striking geodes are those 

 of siliceous composition, which occur usually in shaly, fossiliferous 

 strata. The author remarks that " fossils lying in the path of surface- 

 waters are subject to a complete replacement of their substance by 

 silica ; when crushed or fractured, fossils may have the breaks lined 

 with a siliceous deposit. In the latter case, continued deposition and 

 the expansive foi'ce of the crystals will result in a hollow, rounded 

 mass or geode, lined with inwardly pointing crystals, and on the 

 outside covered with remnants of the fossil". In the well-known 

 Keokuk geode-bed which occurs in the Knobstone division of the 

 Lower Carboniferous at Keokuk, Iowa, the geodes are not disseminated 

 along the planes of stratification, but they lie on the edge or close to 

 joint planes or rifts in the strata through which water had easy access. 

 Elsewhere they occur in the surface- soil and in the beds of streams. 

 Most of them appear to have been formed in the way above mentioned, 

 and to have originated from Crinoids and Brachiopods. 



Yegetation of Bkitish Peat Mosses. — " The Changes in the 

 Vegetation of British Peat Mosses since the Pleistocene Period " is 

 the title of a paper by Mr. F. J. Lewis (Proc. Liverpool Geol. Assoc, 

 New Series, No. 3, November, 1908). The observations are here 

 confined to. Scottish peat mosses from the Southern Uplands to Skye, 

 the Outer Hebrides, and Shetland. The deposits are grouped as First 

 (Lowest) Arctic Bed, Lower Forest, Second Arctic Bed, and ITpper 

 Forest. Concerning these the author remarks "that the first Arctic 

 bed contains an Arctic-Alpine flora, which existed over wide areas 

 near sea-level, and this is succeeded first by forest with temperate 

 plants, which in turn yields place to a second Arctic-Alpine association. 

 These successive strata indicate a definite sequence of events taking 

 place simultaneously over the whole of Britain". The Upper Forest 

 is not met with beyond the mainland of Scotland, and in areas 

 examined in the Grampians and in Assynt the deposits comprise 

 two zones, separated by peat with remains of Arctic birch and willow. 

 In the south of Scotland the Upper Forest consists for the most 

 part of Scotch pine. In his concluding remarks the author finds it 

 difficult to reconcile these several stages in the peat with the theory 

 of a single glaciation, as they suggest considerable oscillations in 

 climatic conditions, and agree closely with the scheme of classification 

 proposed by Professor James Geikie. Remnants of local ice-sheets and 

 glaciers may have existed in the mountains when the earlier beds 

 were laid down, and it would have been better if the author had 

 used the word "during" instead of "since the Pleistocene period"' 

 in the title of his paper. 



