Chiseldon to Collingboume, 



93 



chalk, and both these divisions of the chalk have thin hard beds 

 and seams of marl, all of which have their due significance. 



The distribution of the fossils in the chalk and the consequent 

 establishment of zo)ies, or bands, in which similar assemblies of 

 fossils occur, and of stages, have received much patient investigation 

 at the hands of Dr. Barrois and Messrs. Jukes-Browne, Hill, and 

 others, and the history of the chalk — once thought to be so simple 

 — is found to be a complicated one. 



We will now commence our journey along the line, starting 

 from Swindon, though this is outside the limits of our diagram. 



The old town of Swindon stands on high ground, a capping of 

 Portland sand and limestone, on clay. The old houses avoided the 

 clay and kept to the sandy beds, for water was found in the sands 

 over the clay. This clay is the Kimeridge clay. It is a very fine 

 clay, and this fineness and its marine fauna show that it was laid 

 down in a fairly deep sea at some distance from a shore line. 



The false-bedded sands of the Portland beds show current action, 

 nearness of land, and re-elevation after the Kimeridge depression. 



As we see the Kimeridge clay at Swindon Station dipping away 

 under the Portland sand to the south, we expect it to disappear in 

 that direction. 



So when we examine the next cutting, near Burderop Wood, we 

 are not surprised to find a fresh bed there. This is a clay, best 

 known as the gault, forming the narrow belt of lower land at the 

 foot of the chalk and upper greensand hills. 



The beds in the diagram are numbered to distinguish them and 

 shaded as well ; the gault is number 2, and the Kimeridge clay 

 number 1. 



Though the Kimeridge clay is shown in the diagram as actually 

 passing under the gault in the railway cutting, this is inferred, as 

 we do not see it doing this, but we know it must be so. 



As the gault clay is not very thick — only about 70ft. — -and is 

 dipping steadily south, it soon disappears under the upper green- 

 sand, and it may be seen doing so in the next cutting to the south. 



The name gault is a looal one for a greasy clay, and the name 

 was used geologically as early as 1788. Grault is a bluish or grey 



H 2 



