Vol. 69.] TWO DEEP BOEINGS AT CALVEKT STATION. 313 



■Clay in the borehole, it can be safely assumed that all beds passed 

 through at Calvert, to the depth of 97 feet 3 inches, belong to the 

 ■ornatum zone. 1 



(ii) Forest Marble. [J. P.] 



The Oxford Clay rests upon a grey, partly oolitic, and somewhat 

 earthy limestone ; but, unfortunately, no specimen which showed 

 the nature of the junction was obtained. The tough shelly clay 

 with Cosmoccras (described above) occurred, however, at the actual 

 base of the ornatum beds in the borehole, and no trace of any 

 conglomerate was found. The limestone on which the clay reposed 

 was uufossiliferous, and its inclusion in the Forest Marble is made 

 on lithological grounds. In appearance the bed is similar to the 

 limestones associated with the green clay. Dr. Davies, who ex- 

 amined this and other limestones under the microscope, gives 

 details of its structure on p. 318. It is oolitic, and is thus quite 

 unlike any of the beds of the Cornbrash ; moreover, it passed 

 gradually downwards into limestones which contained Forest- 

 Marble fossils, so that there can be no doubt that it is correctly 

 included in that formation. Thus there is a marked non-sequence, 

 iboth the Ivellaways Rock and the Cornbrash being absent. 



The details of the Forest Marble at Calvert show that the upper 

 portion of the group is more calcareous than the loAver, and in this 

 respect they agree with the descriptions of the various sections of 

 the formation exposed in the Bicester neighbourhood. The lime- 

 stones in the core differ considerably in texture and purity : some 

 beds are bluish grey, oolitic, and shelly ; Avhile others are softer, 

 darker, and earthy, and frequently contain irregularly-shaped lumps 

 of pale-grey clay. It was noticed also that many tube-like hollows, 

 each rilled with a marly and markedly-oolitic clay, traverse the 

 beds of some of the more clayey limestones. The lower portion 

 of the group is chiefly composed of grey, brown, and greenish clays, 

 with a bed of greenish sandstone. Unfortunately, the cores were 

 badly obscured at this point, and the details were somewhat difficult 

 to secure, but those given on p. 310 probably represent the correct 

 order of their succession. The occurrence of green or greenish 

 •clays in the Forest Marble is characteristic. They are met with 

 in all the sections of the formation that are exposed in the railway- 

 cuttings between Blackthorn Hill and Ardley Wood, near Bicester, 



1 [The term 'ornatum zone' is here used in its wide sense: at least two 

 horizons can be distinguished within it in England —a higher duncani zone 

 with pyritized ammonites, seen at Sunnnertown (Oxford) and many places in 

 the Peterborough district; and a lower zone with crushed ammonites, known 

 at Christian Malford, and Dogsthorpe (Peterborough). It is the latter zone 

 which is found at Calvert : it is usually termed the jason zone, but this is an 

 unsatisfactory name, as the identity of the index-species is uncertain. Between 

 these two zones in France and South Germany lies the castor and pollux zone, 

 with a fauna unknown in England. See L. Renter, ' Die Ausbildung des 

 Oberen Braunen Jura im Ndrdlichen Teile der Frankischen Alb' Munich, 

 1908, pp. 75-81.] 



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