Reviews — CD. WakoWs Fossil Meduscs. 81 



I^ E "V I E "W s. 



I. — EossiL Medusa. By Charles Doolittle Walcott, F.G.S. 

 Monographs of the United States Geological Survey, Vol. XXX. 

 4:to; pp. 201, pis. 48, figures in text 26. (Washington, 1898.) 



THE main interest and importance of this Monograph on Fossil 

 Medusa depend on the descriptions given of some peculiar 

 bodies, regarded by Professor Walcott as casts of Discerned usse, 

 which occur in considerable numbers in shales, of Middle Cambriiin 

 age, in the Coosa Valley, Alabama. These bodies are found in 

 intimate association v^rith siliceous nodules, sometimes apparently 

 constituting the entire nodule, at others in solid projecting relief on 

 the outer surface, and also entirely enclosed and surrounded by the 

 mineral constituents of the nodule. 



The semi-cherty nodules, or star-cobbles as they have been termed, 

 in allusion to their radiately lobed forms, which suggest a rude 

 resemblance to starfishes or sea-urchins, occur as narrow bands 

 in the finely laminated shales ; and they weather out on the surface 

 and in the drainage channels of the district in such abundance that 

 9,000 specimens were collected for study by the United States 

 Geological Survey, but only about one-fourth of these show the 

 presumed medusa, and many seem to have been formed inde- 

 pendently of the presence of any organisms. Other nodules, 

 however, contain fragments of the characteristic fossils occurring in 

 the shales, such as brachiopods, trilobites, and in a few instances 

 casts of hexactinellid sponge spicules ; the trilobite tests are now- 

 replaced by crystals of quartz. Further, the nodules are traversed 

 in all directions by fine annelid borings, which penetrate the matrix 

 and medusae casts equally. 



, The medusoid nodules have been carefully examined micro- 

 scopically and chemically. Professor Iddings reports that they 

 consist of a mixture of granular quartz in allotriomorphic grains, 

 occasional flakes of muscovite mica, and a small amount of calcite, 

 gas pores, and colouring matter, probably carbonaceous. Dr. Hayes 

 states that thin sections under low microscopic powers show a cIo.se 

 resemblance to ordinary cherts. They have a finely mottled-grey 

 appearance in polarized light, the extinction being similar to that of 

 cryptocrystalline or chalcedonic silica. The chief differences between 

 the nodules and ordinary chert, when seen under a magnification of 

 100 diameters, are the presence in the former of mica flakes, in 

 more or less abundance, and the absence of rhombohedral cavities, 

 usually found in the chert concretions formed in a calcareous matrix. 

 But under a power of 400 diameters Dr. Hayes found a marked 

 distinction between the nodules and chert; the former seemed to be 

 almost wholly composed of two kinds of extremely fine grains with 

 sharply defined outlines. One kind is original detrital quartz, the 

 other a hydrous silicate of alumina or clay. If amorphous silica 



DECADE IT. TOL. YI. XO. II. fi 



