[STANSFIELD] CONCENTRIC RIDGES 119 



this case the skeleton is completely replaced by silica and the outer 

 surface of the test shows the concentric structure, in places. Looking 

 into the calyx the septal edges are seen in some parts, but at other 

 points the continuity of the septa has been destroyed by the develop- 

 ment of similar concentric ridges upon this upper surface of the exposed 

 calyx. The appearance of this specimen is particularly striking. 



The second example is a plate of chalcedony from between Irvine 

 and the Cypress Hills, Alberta.^ There is no note on the accom- 

 panying label to elucidate its mode of occurrence. The plate is about 

 one-tenth of an inch in thickness and on both of its large fiat surfaces 

 is a pattern which consists of concentric ridges similar to those des- 

 cribed above. In this case they are more regular than those found in 

 the fossils, and the largest set of rings shows a distance of half an inch 

 from the centre to the outer limit of the ridges. They are not found 

 equally developed on all sides of this particular centre, however, as 

 interference takes place from ridges developing around other centres. 

 The patterns on the opposite faces of the plate correspond in a general 

 way, but not absolutely, some of the detail being slightly different. 

 A photograph of one side of this plate is reproduced in Plate 1, fig. 1. 

 In the collection, with this plate, there are some bun-shaped pieces of 

 chalcedony, with approximately flat surfaces on one side, showing 

 concentric ridges, while the convex surfaces are smooth. Another 

 double-sided plate seems to be made up of two sets of similar individ- 

 uals to those last mentioned. It is not completely solid and cavities 

 are present, between the convex surfaces of the bun-shaped masses. 

 In some cases the ridged surfaces are not flat, but angular as though a 

 fragment of rock had been in position against it before it solidified. 

 Ridges on the inclined parts of these surfaces are correspondent with 

 each other. 



Somewhat similar ridges to those described above are often seen 

 on the surfaces of chalcedony masses which have been formed in vugs 

 or gas cavities in lava flows, and some of these masses show agate 

 structures within. 



iSee also Harrington, B. J., Can. Rec. Sci.. Vol. IV., No. 2, 1890, pp. 97-99. 



