JOWETT AND MUFF : GLACIATION OF BRADFORD, ETC. 201 
rock which Hes to the north-west of it. At Skipton it is commonly 
a very tough, dark-blue clay, weathering at the top to a brownish 
colour. In the Coal Measure and Millstone Grit areas the blue 
boulder-clay is covered by a variable thickness of yellow boulder- 
clay, the junction being very irregular. In most sections the 
blue clay merges gradually upwards into the yellow clay, but 
sometimes the passage from one to the other is very rapid. 
At the Thornbury Brickworks, on the eastern outskirts of 
Bradford, there is a cutting about 60 feet deep in boulder-clay. 
The upper ten feet consist of jointed yellow clay with boulders, 
which rests on tough blue clay, the junction plane being irregular. 
The difference in colour between the yellow clay above and the 
blue clay below is most marked, and yet, when the junction 
was examined, lenticular beds of sand were found to pass quite 
undisturbed across the junction. The upper part of one lenticle 
was in yellow clay, and the lower part of the same in the under- 
lying blue clay. It is impossible to conceive that in this case 
the yellow clay can be anything but the blue clay altered in 
appearance by weathering subsequent to its deposition. The 
junction cannot mark a break in the accumulation of the two 
clays. 
In January, 1900, the cuttings on the Midland Railway north 
of Bingley were being widened, and blue boulder-clay was ex- 
posed. Three months later the same sections, on which the 
marks of the workmen's tools were still visible, were found to 
have weathered to the depth of an inch from the surface to a 
yellowish-brown colour. 
The boulder contents of the two kinds of clay, with the 
exception of limestone boulders, are identical. The few lime- 
stones that have been observed in the yellow clay almost always 
have corroded surfaces, and branching masses of Syringopora 
and Lithostrotion from which all the calcareous matrix has been 
removed have been found in it. These facts show that the 
yellow clay has undergone partial or entire decalcification by 
percolating waters, and the yellow colour of the clay is doubtless 
due to the oxidation of the iron contained in it. 
The boulder-clay generally becomes thinner and more sandy 
in the upper parts of the tributary valleys towards the main 
