xi. FOREST MARBLE. 153 



UPPER DIVISION OF THE GREAT OOLITE GROUP. 

 Bradford Clay and Forest Marble. 



The great mass of oolite in the vicinity of Bath is covered by 

 pale blue clays and thin beds of shelly rock. The former is named 

 Bradford Clay, from the place in Wiltshire where it was most known 

 and best exhibited; the latter is called Forest Marble, from the 

 Forest of Wychwood, where it was first observed by Smith. 

 Taken together, they constitute a variable zone of separation between 

 the Cornbrash and Great oolite, as these terms are commonly under- 

 stood ; variable in total and relative thickness, in structure, and 

 in original circumstances of deposition. They appear to be most 

 frequently marine deposits ; but in many cases, drifted oysters and 

 fragments of wood, and in a few examples Cyrenae, indicate estuarine 

 fluctuations and inflowing fluviatile currents. 



The shelly beds are sometimes compact enough to deserve the 

 title of a rude marble, which may still be seen in ancient farm and 

 manor houses, but it is not now anywhere polished on a large scale. 

 These beds are very irregular, and even within short distances vary 

 greatly in thickness (as at Islip and Kirtlington Station), and 

 admit of much variety in the direction of bedding, from horizontal 

 and parallel to the usual inclination of steep shell drift (30 or 40 

 from the horizon), and this in different directions. 



The clays are not traceable continuously ; they are considerable 

 about Cirencester and Fairford, and are observed as white, partly 

 indurated marl, in the productive excavations of Islip, where 

 Terebellaria ramosissima and Terebratula digona remind us of the 

 Berfield pits at Bradford, but no Apiocrinites have as yet occurred 

 in our Oxfordshire sections. 



One of the most extensive and characteristic excavations in the 

 Forest marble is at Poulton, between Fairford and Cirencester. 

 Here quarries, worked for ages over a large space of ground, have 

 yielded roofing-slate for most of the houses in Lechlade, Fairford, 

 Cirencester, and the neighbouring parts of South Gloucestershire, 

 as well as flags for pavement, and abundance of road materials. 

 The strata lie with a southward slope, and occupy a considerable 

 breadth, without any over-covering of cornbrash; 



