64 G. W. Lamplugh — The Yorkshire Boulder-clay. 



As shown above, other irregular beady streaks of similar sand 

 occur both above and below the main seam, but these are quite of 

 minor consequence, and, when traced, are found to fall into the 

 larger bed. 



Surrounding these sandy seams there is a variable thickness, from 

 15 to 40 inches, of tenacious blue or greyish clay which differs in 

 many respects from the Boulder-clay which borders it. It carries 

 very few pebbles, and, though full of shearing-planes, shows distinct 

 traces of stratification and lamination, and is altogether clearly an 

 aqueous deposit. 



Similar clays are also found at Bridlington and Dimlington asso- 

 ciated with the sandy shell-beds, and sometimes themselves bear 

 shells. These I regard as glacial muds formed on the same sea- 

 bottom as the shelly sand, and probably in sequence with it, which 

 have been removed and carried forward along with the sand. In 

 former cases I thought the clay had been deposited before the sand, 1 

 but in this instance the beds have been rolled over and folded 

 upon themselves, and it is impossible to make out their' original 

 arrangement. 



Besides shells, the sand contains a few small, well-worn pebbles, 

 not generally larger than beans, and these are not chalk or flint, 

 but usualty of a dark, close-grained igneous rock, or otherwise of 

 yellow quartz, though many other varieties are present. The sand 

 itself is chiefly .made up of rather coarse quartz-grains, and 

 altogether the bed is not of such a nature as would be likely to 

 form on a floor of chalk. 



A neighbouring streak, containing many crushed shells, consists 

 almost altogether of these small dark pebbles, with but little sand. 

 The state of preservation of the shells deserves especial attention, 

 as there is in this case such unmistakable proof that the bed has 

 been transported bodily, and has undergone a considerable amount 

 of shearing during the process. 



In the minor streaks the shells ai'e always reduced to fragments, 

 and in the main seam also the majority are broken, though not so 

 shattered. 



In this seam, however, a small proportion occur in perfect con- 

 dition, the bivalves even in some cases having their valves united. 

 I obtained many specimens of Astarte compressa, which is the 

 commonest fossil of the bed, in this condition, and also of Astarte 

 borealis, some fine examples of the latter measuring over an inch 

 across the valves. 



When the shells are broken, the fragments occasionally still lie 

 close together, though oftener they have been trailed apart during 

 the shearing of the bed. As showing the effect of this shearing, it 

 is most interesting to find instances in which the two valves of an 

 Astarte have been displaced or separated without fracture, thus clearly 

 proving a differential motion. These specimens also illustrate a 

 curious feature noticeable in the Boulder-clay, wherein detached valves 

 of various species may be found retaining a small pinch of sand under 

 1 Q.J.G.S. vol. xl. p. 317. 



