Vol. 67.] THE AVONIAN OF BTTRKTNGTON COMBE. 347 



A peculiar fault-breccia, consistiug of pieces of limestone set in 

 a very ferruginous matrix, occurs immediately below this slightly 

 dolomitic band, but is very poorly exposed. It is a noteworthy point 

 that there are signs of disturbance accompanied by the occurrence 

 of unusual minerals — in this case nuor, at the corresponding horizon 

 in the Great Quarry of the Avon section. 



The basal beds or 8 2 seen in the quarry are strongly oolitic and 

 often finely banded limestone, which alternates with fine-grained 

 grey limestone not obviously oolitic in hand-specimens, but com- 

 monly shown by thin sections to be so. Crinoids are also an 

 important constituent of these limestones. Penecontemporaneously 

 brecciated limestone (3) occurs, similar to that described above at 

 (20) and below at (111). (See PI. XXVIII, fig. 4.) 



Lowee Seminula Zone (S x ). 



Extent. — From the prominent rib of rock near the middle of 

 Quarry 2, past the end of the quarry as far as Plumley's Den. Also 

 the prominent rock-mass on the western side of the Combe, as far as 

 the cleft known as the ' Eock of Ages,' which is due to weathering 

 along the same band as is Plumley's Den. 



Thickness.— 130 feet. 



The prominent rib of rock which it is convenient to take as 

 marking the boundary between S x and S 2 consists of thinly-bedded 

 and partly dolomitized limestone-bands alternating with thin shaly 

 partings. The dolomite in this case was probably due to local 

 infiltration. The main part of S x consists, however, of rather 

 thickly-bedded, fine-grained, grey limestone, as a rule obviously 

 oolitic, and probably oolitic even when this character is not visible 

 in a hand-specimen. Fine banding is frequent. 



Further Account of the Lithology of the S x Beds. 



Though the great majority of the limestones in S x agree in being 

 strongly oolitic and foraminiferal, they show a considerable amount 

 of variability. Some (6) are almost entirely composed of small 

 and uniform oolitic grains ; others (111) are very similar in cha- 

 racter to the variety described above at (20), consisting of rounded 

 patches of limestone, sometimes horny, sometimes oolitic, which 

 have clearly been produced by the brecciation of pre-existing beds — 

 succeeded in the first place by a more or less complete rounding of 

 the edges of the resulting fragments, and in the second place by 

 the deposition in certain cases of successive coats round the frag- 

 ments in concretionary fashion, followed finally by the union of the 

 whole in a cement of crystalline calcite (see PL XXVIII, fig. 4). 



Other rocks again from this level are partly oolitic, partly of 

 the brecciated type of limestone just described. A considerable 

 amount of petroleum occurs in the limestone at the level of 

 Plumley's Den and of the cleft in the ' Pock of Ages.' This feature 

 may be observed at the same level in the Avon section. 



2b2 



