32 Field Columbian Museum — Geology, Vol. III. 



A series of 32 specimens received later confirms the characters of 

 the earlier lot. They include a number of globular specimens which, 

 however, have the same structure as the rosette forms, from which 

 they differ in the number and dimensions of the component plates. 

 That is, the globular forms are merely thick rosettes. One specimen 

 consists of a group of many nearly globular forms enclosed in the 

 weathered matrix which assumes the form of a red sand. This sand 

 appears to be the residue left from solution of the limonite cement of 

 a ferruginous sandstone. 



The rosette appears upon both sides of an approximately octa- 

 gonal plate which may be designated the basal plate of the aggregate. 

 This is penetrated obliquely by a variable number of similar plates 

 which appear to intersect at the centre of the aggregate and project 

 on both surfaces. These plates make angles of approximately 30 

 with the bases. While these plates appear as if passing through 

 the basal plate and any important one appearing on one side 

 may be readily discovered on the other, yet the two rosettes are 

 never exactly alike. One is always more complex than the other and 

 formed of smaller plates. These plates generally, but not always, lie 

 in a confusedly whorled position. They are not simple but frequently 

 consist of two plates inclined to each other at angles of approximately 

 30 and intersecting some in the vertical and some in the horizontal 

 plane. By repetition of this compounding of plates, always at 

 angles of approximately 30 so far as the roughness of the 

 material will allow determination, the apparently irregular orientation 

 of the leaves of the rosettes may be accounted for. By a greater 

 degree of this compounding also is the greater complexity of one face 

 over the other produced. The specimens, examined detail by detail, 

 are decidedly unsymmetrical, yet when the broader features only are 

 considered, symmetry of a high order is present. The rosettes on 

 either side of the basal plate while not identical in detail are so in 

 mass, and proportioned so that the aggregates are symmetrical with 

 respect to the plane of the basal plate, as well as to a central axis at 

 right angles to this plane. There is also a tendency in some of the 

 specimens towards an axis of hexagonal symmetry in the plane of the 

 basal plate. The secondary plates appear to so twist as to all intersect 

 along this axis. 



The position of those portions of the plates which lie buried in 

 the body of the specimen may be followed by the cleavages upon the 

 fractured surfaces. From an examination of these cleavages it 

 becomes evident that the plates do not really intersect or interpene- 



