Vol. 67.] THE AVONIAN OF P.TTRRINGTON COMBE. 349 



to compare the relative resistance of these several constituents to 

 the process of dolomitization. The first material to be affected 

 is the fine calcareous matrix, then the foraminifera are attacked ; 

 while the pieces of crinoid stem formed of crystalline calcite are 

 highly resistant, and may remain unaffected when the whole of the 

 surrounding material is dolomitized. 



As regards the conditions of formation of these dolomites, the 

 facts appear to be in conformity with the now generally accepted 

 view — that they originated by the practically contemporaneous 

 alteration of limestone under shallow-water conditions. It is a 

 well-established fact that shallowing of the water of the Car- 

 boniferous Limestone sea took place throughout the main part of 

 the South-Western Province in Middle Avonian times; and the 

 occurrence of these dolomites on the same general horizon as the 

 shallow-water beds elsewhere, is evidence that the shallowing 

 extended to the Mendip area. Below the dolomites is a highly 

 fossiliferous series forming the remainder of C 1 and y. The lime- 

 stones of this level are entirely non-oolitic, and are in the main 

 fine-grained and dark grey. Corals, brachiopods, and especially 

 crinoids abound, and sections show polyzoa and foraminifera to be 

 frequent. 



The name ' Petit Granit ' is employed by Belgian geologists to 

 designate a rock mainly made up of larger crinoid - fragments 

 embedded in a matrix of smaller ones. It occurs commonly in 

 the 7 beds and in the upper part of Z (see PI. XXIX, fig. 2). 



Upon the cherts, which are very prominent, the formation of the 

 splendid scarp of the upper part of the Combe partly depends. 



Upper Zaphbextjs Zone (Z 2 ). 



Extent. — The lower slopes of the Great Scarp and the pro- 

 jecting mass of rock at the bend of the Combe. 

 Thickness.— 140 feet. 



Lithologically the Z 2 beds closely resemble the y beds : differing 

 in the absence of chert-bands ; in the occurrence of shaly partings ; 

 and in their thinly-bedded character, so that they weather easily. 

 They consist of dark-grey, generally crinoidal and highly fossiU- 

 ferous limestone, much being of the ' Petit Granit ' type. 



Lower Zaphrextis Zone (Z x ). 



Extent. — The main part of the exposures in the valley of 

 the western twin-stream, from the Goatchurch cave to the road ; 

 and the corresponding series in the eastern twin-stream. Almost 

 all the southern face of the upper part of the Combe, except just 

 at the bend, is formed by Z x beds. 



Thickness.— 140 feet. 



A type of limestone similar to that in Z prevails also in Z,. 



