444 PRINCIPLES OF STRATIGRAPHY 



knolls, they are commonly inclined, and an interlapping of the clastic 

 layers with layers composed of Membranipora and Serpula masses 

 is common. 



The reef itself consists of a cavernous limestone composed of 

 knolls of ]\lembranii)ora with the intervening areas largely filled by 

 a small Spirorbis shell. The surface is commonly marked by wind- 

 ing channels and irregular depressions, in which Maotic mollusc 

 shells abound. Especially numerous is the shell Sphenia cimmeria 

 Andrus. which makes up masses of shell limestone, sometimes in 

 cavities within the reef itself. The sides of the reef masses are 

 commonly very steep, often vertical and at times overhanging, and 

 the surface of the reef at all stages of growth possessed a higher 

 hypsometric * niveau than did the surrounding bedded material. 

 The height of the reef masses ranges up to 30 meters. Sometimes 

 they are embedded in argillaceous layers, derived probably from the 

 neighboring land. 



Bedded Reefs. 



A type of reef differing from the lenticular one described is found 

 in a number of Palaeozoic rocks, where stromatoporoids are the 

 chief or only reef organism. Instead of forming local aggregations 

 built up into lens-like mounds, the heads of the stromatoporoids are 

 evenly distributed in beds of slight thickness, but of wide horizontal 

 extent. They grew apparently on a shallow sea bottom, at a con- 

 siderable distance from the shore, as shown by the frequent worn 

 character or overturned position of the heads, and by the scarcity 

 or absence of siliceous sediment. Sometimes the reef consists of 

 a single layer of "heads," which may measure up to 5 or 6 feet in 

 diameter; at other times it consists of superimposed heads or frag- 

 ments of heads, forming beds twenty or more feet in thickness and 

 traceable for several thousand feet or even, with some interrup- 

 tions, for miles. Sometimes this sheet reef contains only a small 

 number of heads more or less worn, and embedded in calcarenyte 

 derived from the wear of the stromatoporoids, and sometimes it is 

 entirely replaced, for a short distance, by the coral sand. Chan- 

 nels, filled with the coral sand, are characteristic features, and 

 these and the larger interruptions of the continuity of the reef pro- 



* Andrussow has proposed the term Palreohypsometric to express the relative 

 position which the deposits had at the time of formation, while folding and dis- 

 location may have entirely changed this relationship, the present hypsometric 

 position of such deposits not being the same as that which they possessed at 

 the time of their formation. 



