648 E. Blackwelder — Reef of Calcareous Algw 



lated although closely spaced. Since the shale is not in the 

 least cemented to the limestone it can be removed completely, 

 leaving a clean limestone surface. This fact also suggests that 

 the limestone domes were already solid and stony when the 

 mud (now shale) began to be sifted over them in the water of 

 the Cambrian sea. The fortunate removal of the friable shale 

 matrix permits a satisfactory examination of the masses in 

 three dimensions. Furthermore, many of them have been split 

 open by the frost, and hence the internal structure can now be 

 seen. All that is left to be desired is that these inner sections 

 might be polished so that their minute structures would become 

 visible. 



On examining a single mass or colony, it is found to consist 

 of a thick outer shell made of globular masses about the size 

 and shape of a cabbage, closely packed together and cemented 

 by small quantities of dense blue-gray limestone which has pre- 

 cisely the appearance of mortar in a stone wall (see fig. 2). 

 The resemblance of the tumor-like bodies to cabbages is fur- 

 ther enhanced by the fact that they show a distinct concentric 

 structure, or at least a hemispherical banding parallel to the 

 outer surface. An attempt has been made to show this in fig. 

 3, since photographs in the field were unsuccessful in recording 

 the rather obscure internal structure. Inside of the mam mi 1- 

 lated shell, but intergrading with it, there is a vaguely dis- 

 tinguished central portion consisting of dense limestone of 

 slightly mottled gray color, in which several concentric shells 

 are tinged with ocher yellow, — doubtless because of the pres- 

 ence of ferrous carbonate in the limestone. This central band- 

 ing, although roughly concentric, is slightly wavy in sympathy 

 with the mammillary structure of the outer part. 



A closer study of specimens taken from different parts of 

 the mass, carried on with the aid of the microscope and chemi- 

 cal tests, shows that all but certain portions of the interior 

 now consist of dolomite rather than limestone. As anticipated 

 from the very common association of iron oxides with dolo- 

 mite, the ocherous portions are exclusively dolomitic, but they 

 enclose more or less continuous layers of dense gray limestone. 

 The dolomitic part has either a uniform or a finely-brecciated 

 structure, each little rhomb being coated with limonite. On 

 the other hand, the limestone bodies which seem to represent 

 the original portions of the rock show vague layers and also 

 many circular and elliptical sections which may have been 

 originally canals or tubes. They are not brecciated. Inside of 

 these limestone portions there are also obscure, more or less 

 branching bodies, colored a dark sepia brown, apparently by 

 bituminous matter. The cabbage-shaped bodies of the outer 

 shell consist entirely of dolomite of a dark smoky gray color, 



