UPPER, OR FLINTY CHALK. I43 



XIII. 

 II. 8. UPPER, OR FLINTY CHALK. 



The Upper Chalk is characterized by its numerous parallel layers 

 of flint. In this county it constitutes by far the most considerable portion 

 of the chalk formation, extending to the summits of the highest hills, and 

 in some instances reaching nearly to their base. 



The chalk of this deposit is generally of a purer white, and of a softer 

 texture, than the inferior strata; but in other respects, presents no sensible 

 diiFerence. It is regularly stratified, and partakes of the general inclina- 

 tion of the other divisions of the series. It is separated by horizontal 

 layers of siliceous nodules into beds that vary from a few inches, to 

 several feet in thickness, and which, in some locahties, are traversed by 

 obliquely vertical veins of tabular flint, that may be traced for many 

 yards without interruption. These are sometimes disposed horizontally, 

 and form a continuous layer of thin flint, of very considerable extent. 



The nodular masses of flint are very irregular in form, and variable in 

 magnitude ; some of them scarcely exceeding the size of a bullet, while 

 others are several feet in circumference. Although thickly distributed in 

 horizontal beds or layers, they are never in contact with each other, but 

 every nodule is completely surrounded by the chalk. Their external sur- 

 face is composed of a white opaque crust, consisting of an intermixture 

 of chalk and silex, probably formed by a combination of the outer surface 

 of the nodule with its investing matrix, while the former was in a soft 

 state. Internally they are of various shades of grey, inclining to black, and 

 often contain cavities lined with chalcedony and crystallized quartz. 



When first extracted from the quarry, flint is brittle, has a conchoidal 

 fracture, and feeble lustre ; thin fragments are translucent. Its specific 

 gravity is 2-594. According to the analysis of Klaproth, it consists of 



