Algonkian Algce, in Penman of England. 205 



the rock, the only possible explanation other than that 

 the structures are due to secondary ^'concretionary" 

 changes in the sedimentary mass, is that we have here 

 structures that might be compared to stromatolites and 

 oolites— primary deposits, yet not of a character that 

 would justify the introducing of generic and specific 

 names. Yet there are fundamental differences between 

 oolites and the specimens shown in figures 1 and 3, and 

 between stromatolites and specimens like those in figures 

 2, 5, and 6. First among these differences is the great 

 thickness of the laminae when compared to the delicate 

 laminations of oolites and stromatolites (the corre- 

 sponding hot-spring and ^'sinter" calcareous deposits 

 included) ; another is the very considerable dimensions of 

 the spherical bodies here shown, a parallel to which is 

 not kno^^m from recent deposits. In addition, we see that 

 the spherical bodies often pass with their lamination into 

 each other, as may very distinctly be seen in parts of the 

 specimen depicted in figure 3, and even more so on the 

 other side of the specimen. The spheres are parts of a 

 large coherent mass. This feature contrasts decidedly 

 with the ordinary oolitic structure, but is not infrequently 

 seen in typical concretions, e. g., in chert nodules where 

 the laminae may be arranged around different centers, 

 yet the outer laminae of one sphere pass into the corre- 

 sponding ones of the neighboring spheres. 



In the botryoidal concretions so common in the Fulwell 

 quarries, we have great clusters of coherent spheres that 

 suggest a somewhat similar mode of formation. A most 

 important feature of difference is the existence of a some- 

 times extremely strongly marked radiating element in the 

 Permian structures ; a similar type of radiating rods is 

 not known in stromatolites and oolites. When there is a 

 radiating structure in oolites, it is due to crystal fibres, 

 while the rods of the structures here figured are made up 

 of fine-grained — sometimes extremely fine-grained — lime- 

 stone. We may have tube-like structures in the stromato- 

 lites, indeed these are very typical, but these tubes are, 

 when the rock is in an unchanged condition, not at all of a 

 similar regular type.^^ 



Thus, judging from the general characters of the speci- 



^^ How a secondai"}^ chemical change of the rocks tends toward giving- a 

 more regular, geometrically arranged pattern is illustrated in the regularity 

 of the silicified tubes in the stromatolite layer from the Alten district! 

 figured in my article on Finmarken of 1919 (p. 93), when compared with 

 the figure on page 91 of the same paper wheh shows the typical irregularly 

 tubular stromatolite structure of an unaltered dolomite. 



