200 0. Holtedahl — Structures like Walcott's 



or partitions represent the deposit made by the algae and 

 are now a buff-colored and grey magnesian limestone. 

 The ends of a group of the tubes filled in with the lime- 

 stone appear like a group of miniature basaltic columns, 

 and the base or lower side of the same tubes has irregu- 

 larly oval and round, concentrically marked forms that 

 appear to be the filling of the ends of the tubes . . . The 

 walls are arranged in echelon and the fillings break out as 

 plates of columns." 





mmmmr^' ■ -•"■^ 



'-^-:^^^:^^^^^s>i*.^^^;^mmm 





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ai^ll 



m 





^^il 



p^' 



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iP-W.i ^ ^^^i 



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y'Ua 



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M 



Fig. 6.— Three-fifths nat. size. 



As to the mode of growth, AValcott states: "As far as 

 indicated by the specimens collected by Mr. M. Collen the 

 cellular structure grew with the tubes more or less paral- 

 lel to the bottom and in some instances upright or at right 

 angles to the bottom." Corresponding to this type we 

 have the "irregular cylindrical or conical pillars" of 

 Sedgwick and the "basaltiform" type of Abbott. I have 

 a specimen of this type and it is shown in figures 4 and 5. 

 The likeness between figure 4 and Walcott's cross- 

 sections of G. hasaUica (pi. 17, fig. 2; pi. 18, figs. 1, 2) is 

 very striking indeed. In the Fulwell quarries the tubes 

 also occur at very different angles. Walcott's genus 

 Copperia likewise belongs to this type of structure. 



As to Camasia, I have no specimen that can be directly 

 compared with those of C. spongiosa figured by Walcott. 

 It should not be difficult, however, to find this type in the 

 Fulwell quarries if it were looked for especially. In 



