82 FIRST REPORT - 1881. 
hibited all the inflexions of the fibres and the dimensions of 
their diameter as they approached to their termination. 
In the greater number of lenses the structure is perfectly 
symmetrical in relation to the anterior and posterior surfaces, 
or to the poles of the axis of vision : but Dr. Brewster dis¬ 
covered in some lenses a remarkable deviation from this sym¬ 
metry ; the anterior surface having the fibres arranged accord¬ 
ing to one law, and the posterior surface according to another. 
The object of this singular structure he conceived to be to 
obtain a more perfect correction of the spherical aberration of 
the eye to which it belongs. 
Mr. Murchison, President of the Geological Society, com¬ 
municated, verbally, observations on certain accumulations of 
clay, gravel,marl, and sand around Preston in Lancashire,which 
contain marine shells of existing species. 
The marine shells of existing species in this district were 
first noticed by Mr. Gilbertson of Preston ; and Mr. Mur¬ 
chison was desirous of calling the attention of the Meeting to 
the merits of that able naturalist. 
He had this year visited the localities, and found the deposit 
in question to consist, near the surface, of clay with boulders 
of distant rocks, covering great thicknesses of marl, gravel, and 
sand, the sand usually being the lowest. These accumulations 
are not only spread over the broad delta extending from the 
coast at Blackpool to Preston in the interior, but they rise at 
the latter place into considerable eminences extending in pla¬ 
teaux on the banks of the Ribble and the Darwent, for several 
miles inland. 
In certain places the marls, sands, and gravels contain shells 
of existing species (Mr. Gilbertson enumerates about 20 spe¬ 
cies), not differing from those of the adjoining sea, above which 
they were found at various heights from 80 to 300 feet. The 
accumulations seldom offer proofs of regular bedding or tran¬ 
quil deposit, but rather resemble the detritus formed upon an 
agitated shore ; although from their diversity of structure they 
present distinct evidence of having been heaped up during a 
long protracted period. Seeing the height above the sea at 
which these shells are found, and that they are usually buried 
under a cover of clay, containing large boulders of Cumbrian 
rocks, Mr. Murchison infers that one of the last elevations of 
the central ridge of the North of England is thereby proved to 
have taken place after the creation of existing species of 
animals. 
