Algonkian Algcc in Permian of England. 203 



concretionary limestones were originally dolomitic lime- 

 stones, and there can be little doubt that these rocks were 

 impregnated with sulphates and that the solution of these 

 brought the bed into a condition which gave the concre- 

 tionary forces full play; and further it would seem 

 highly probable that the complicated structures occurring 

 in these rocks were mainly produced when the beds were 

 saturated in solutions of calcium sulphate containing 

 colloidal organic matter. It is the presence of this latter 

 matter that causes the spheroidal condition. "^^ 



From a study of the few specimens at hand, the inor- 

 ganic and secondary character of the structures seems 

 also to be evident. What is observed at once, both in the 

 field and in the literature, is that there generally is in 

 these structures every transition between the various 

 types, and that hence they can not well have the value of 

 "genera" and "species." Abbott, who speaks of the 

 "hundred or more patterns met with," like Woolacott, 

 distinguishes several main types (Abbott's honeycomb 

 and coralloid; Woolacott 's spherical and cellular), and 

 these again each divide into minor varieties, but that 

 is all that can be done with them. We may find any 

 sort of variation and transition. As Sedgwick says, 

 there is no end to the modifications. The thickness 

 of the laminae varies, and the distinctness of the 

 radiating structure as well. In figure 6 very different 

 characters can be seen in this one specimen. In the 

 lower part, only the concentric laminae are seen, while 

 in the upper left-hand part, radially arranged tubes 

 are very distinct. These tubes or rods come into 

 existence by more or less conspicuous thickening of 

 the concentric laminae at regular intervals. The tubes 

 can also be very slender, and the concentric laminae 

 only slightly indicated, as in figures 7 and 8 (top). 



One feature that does not vary, however, is the arrange- 

 ment of these radiating rods or indications of rods 

 vertical to the concentric laminae. Where the lamination 

 curves, the directions of rods change correspondingly. 

 In the upper part of the specimen illustrated in figures 

 4 and 5, the tubes are straight and parallel, and here we 

 find a totally regular lamination, with the laminae in a 



^° The possibility of a primary deposition has, however, been mentioned 

 by Jukes-Browne (Geol. Mag., 1891, p. 528), and B. A. Green has suggested 

 (see Woolacott 1912, p. 270) that the rock was originally a tufaceous 

 deposit, the concretions being in part due to irregular precipitation, and in 

 part to later rearrangement. 



