abstracts: geology 563 



and many data are given relative to localities, depths of water, and 

 bottom temperatures. The crustaceans, some brachiopods, some 

 alcyonarians, and a few worm tubes were found to be notably phos- 

 phatic, and therefore to be considered in discussing the origin of phos- 

 phate rock. The foraminifera, alcyonarians, echinoderms, crusta- 

 ceans, and coralline algae, with some sponges and bryozoans are dis- 

 tinctly magnesian, and therefore contribute to the formation of 

 magnesian limestone. Mollusks, millepores, and the madreporarian 

 corals have shells or skeletons of nearly pure carbonate of lime. So 

 far as present evidence goes, the presence or absence of magnesia 

 depends upon the nature of the calcium carbonate, whether it is ara- 

 gonite or calcite. The aragonitic organisms are all, so far as we now 

 know, non-magnesian; all of the magnesian forms are calcitic. Mag- 

 nesium carbonate is isomorphous with calcite, and goes with its natural 

 partner. 



In the magnesian organisms, especially in the series of echinoderms 

 and alcyonarians, a remarkable relation was discovered. In cold 

 water forms the proportion of magnesium carbonate is low, varying 

 from 6 to 9 or 10 per cent, while in organisms from warmer waters 

 it runs much higher, from 10 to 15 per cent. In some algae the per- 

 centage of magnesium carbonate in the inorganic portion of the speci- 

 mens analyzed, was as high as 25 per cent. 



In the general discussion at the close of the paper the application 

 of the analyses to the study of coral reefs is considered. F. W. C. 



GEOLOGY. — Baked shale and slag formed by the burning of coal beds. 



G. Sherburne Rogers. U. S. Geological Survey Professional 



Paper 108-A. Pp. 10, 3 plates. 1917. 

 The baking and reddening of large masses of strata, caused by the 

 burning of coal beds in place, is a striking feature of the landscape in 

 most of the great western coal-bearing areas. In places enough heat 

 is generated to fuse and thoroughly recrystallize the overlying shale 

 and sandstone so as to form natural slag. Some of the slags thus 

 formed resemble igneous rocks both in hand specimen and in thin 

 section but others consist largely of rare and little known minerals. 

 Thoroughly fused slag seems to occur chiefly in crevices or chimneys 

 through which the hot gases gen^ated in the burning escaped through 

 the overlying strata to the surface. It is probable that the ignition 

 of the coal has been due most commonly to spontaneous combustion, 

 especially in places where topographic conditions are favorable. 



