84 THE CAMBRIAN AND ORDOVICIAN DEPOSITS OF MARYLAND 



At first these films are practically horizontal to the bedding planes, but 

 soon undulation commences and narrow or broad folds with narrower or 

 sharper bending down of the films occurs. After an interval of several 

 inches exhibiting such undulation, the horizontal lamination is resumed 

 and this in turn is followed by a repetition of the undulations. These 

 wavy outlines as seen in cross-sections of the strata appear as concen- 

 trically lined areas of varying diameter on the bedding planes themselves. 

 The greater the width of the fold seen in transverse section, the greater 

 the diameter of the corresponding concentric area. 



These laminated strata at the base of the Conococheague follow two 

 distinct patterns. In each the basal laminae are horizontal to the bedding 

 plane, but the succeeding undulations are quite different. In one kind 

 the undulations are an inch or less across and retain this diameter 

 uniformly. In the other, the width of the undulations varies from a 

 central one, several inches across to lateral ones an inch or less wide. 

 Upon the weathering of the surrounding strata, masses of this laminated 

 rock are left in the soil, still retaining their calcareous composition or, as 

 is more frequently the case, changed to silica. In either case the uni- 

 formity in shape of these residual masses would seem to indicate that 

 they are definite organic structures. 



Walcott has described a number of quite similar laminated structures 

 from the Proterozoic rocks of the West and has shown that they represent 

 the secretions of calcareous algae. Certain of the Proterozoic limestones 

 contain beds crowded with these algal structures which are repeated again 

 and again through thousands of feet of strata. These remains are not 

 those of the fossil plant itself, but are simply the secretions of calcium 

 carbonate upon the tissue of the plant. As is well known, calcium 

 carbonate held in solution by an excess of carbon dioxide in the water 

 is deposited when the carbon dioxide is abstracted. In securing carbon 

 from the carbon dioxide for the building of their tissues the lime is 

 deposited upon the films of the plant which abstracts the carbon dioxide. 

 The form of the plant, however, is well preserved in these limestone 

 secretions. 



