226 B. Thompson — Fresh-water Shells in the Lincolnshire Oolite. 



Next come the interesting questions of how and when these shells 

 got embedded in the limestone. To the question how ? we can give 

 a fairly definite answer. The upper part of the Lincolnshire Oolite, 

 for at least as far down as these shells were found, must have been 

 disturbed and re-deposited by water, or ice and water, and the shells 

 introduced at this time ; for although they have been found at four 

 or five places, in none of these was there any crack or pipe leading 

 to the surface, through which they could have made their way down, 

 even supposing such fragile shells could have survived the journey. 



The " pipes " or gullies containing clay and stones have been 

 particularly searched for these shells, but without reward; this is 

 not surprising, for the " pipes " are not wide enough (they vary 

 from a few inches to a foot at the most) for the matter in them to 

 have been precipitated down in mass, even supposing they were at 

 any time open tubes or crevices. Besides, it is pretty certain that 

 these " pipes " were slowly made and enlarged by water percolating 

 from the surface to the springs at the base of the Oolitic series, so 

 that all the material now filling them has gradually got in and 

 worked itself down. Nearly all the flatter stones in them are on 

 edge, a position they would naturally assume in making their way 

 down in the manner indicated, because it is the one offering least 

 resistance to their passage. 



It is unlikely that fragile shells could have survived the pro- 

 tracted journey involved by this explanation, even if introduced at 

 any period, at the top, enclosed in a matrix of clay or soil. It is 

 almost certain, however, that the "pipes" are much newer than 

 the period of the introduction of the shells into the limestone (see 

 below). 



Having arrived at the conclusion that a re-deposition of the 

 Oolitic Beds, as far down as the shells were found, was a necessary 

 corollary to their being so found, a particular search was made for 

 other evidences of it, and we found, without difficulty, here and 

 there in the Oolite, abovt as low dmon as the shells were found, but 

 no loiver, pebbles and small pieces of flint similar to those found in 

 the "drift" above. Very likely the hummocky masses of indurated 

 limestone previously referred to (3) indicate, where they occur, the 

 depth to which the disturbance which permitted of the introduction 

 of the shells extended. 



The question of when? this re-deposition occurred is a more 

 difficult problem to solve. I can find no record of the occurrence 

 of these land and fresh-water shells in any of the Glacial deposits, 

 yet it would appear that they cannot be more recent than the Glacial 

 period. I would offer the following hypothesis on the subject : The 

 animals that formerly inhabited these shells lived in the district 

 near to where they are now found prior to the Glacial period, and 

 their habitations got gradually buried in the soil or clayey subsoil ; 

 from this position they were moved with masses of the underlying 

 rock by the advancing ice, and mixed with material from other 

 districts, thus forming the oldest Glacial deposits of the district, of 

 which scarcely a trace in its original form now exists. During the 



