162 abstracts: botany 



general along the lines of surface scratches, and are apparently unre- 

 lated to the service conditions, and to the direction of rolling of the 

 sheet. They appeared some eight or ten years after the completion of 

 the roof. The tin coating on copper consists of at least three layers; 

 viz., a thin layer of CusSn immediately next to the copper, than a layer 

 of Heycock and Neville's constituent H, containing about 60 per cent 

 by weight of tin, and finally, a layer of the eutectic of tin and copper, 

 in which most probably is found any lead that may have been present 

 in the tinning mixture. The constituents of these intermediate alloy 

 layers are more electronegative than either the tin or the copper. Con- 

 sequently, when the copper becomes exposed, as at the bottom of 

 scratches on the surface, it forms together with the alloy layer a gal- 

 vanic couple, electrolytic action sets in, and the copper at these points 

 is corroded, forming the pits mentioned. P. D. M. 



GEOLOGY. — Relations of the Embar and Chugwater formations in central 

 Wyoming. D. Dale Condit. U. S. Geological Survey Profes- 

 sional Paper 98-0. Pp. 263-270, with 3 plates and 2 figures. 1916. 

 The Embar formation of central Wyoming, which lies between the 

 Tensleep sandstone and the Chugwater formation, comprises several 

 distinct facies, each of which is considered in detail, and some of the 

 formational boundaries in the Bighorn Mountain region are redefined. 

 In connection with the description of the gypsum and associated strata 

 it is suggested that possibly conditions were favorable for accumu- 

 lation of salt beds also. The chance of finding such deposits down the 

 dip below the surface is believed to be sufficient to merit further 

 investigation. R. W. S. 



BOTANY. — Maxonia, a new genus of Tropical American ferns. Carl 

 Christensen. Smithsonian Miscellaneous Collections, 66 9 : 1-4. 

 1916. 

 The type and sole species of this new genus, M. apiifolia, is a rather 

 rare polypodiaceous fern of Jamaica and Cuba, described by Swartz 

 in 1801 under Dicksonia, and since variously referred to Dryopteris 

 (or Nephrodium) and Polystichum. From Dryopteris, in which it was 

 last placed as a special subgenus by Maxon in 1909, it differs in the 

 unique morphology and development of the indusium, as also in its 

 strongly dimorphic leaves and peculiar rope-like rhizome. The last 

 two characters recall Polybotrya, between which genus and a small 

 subgroup of Dryopteris species it is somewhat intermediate. A sub- 

 species, M. apiifolia dualis, occurs in Guatemala. P. C. S. 



