726 
DE. W. B. CAEPENTEE AND ME. H. B. BEADT ON 
examine by the kindness of Mr. Robert Etheridge), the infiltrating material is siliceous. 
In either case it is instructive to examine sections made sufficiently thin to be capable 
of being viewed under the Microscope by transmitted light; but whilst in the specimens 
in which the infiltrating material has been calcareous , it has usually blended with the 
arenaceous framework in a degree sufficient to obscure the precise boundaries of the 
latter, in the specimen solidified by silex each individual sand-grain can be distinguished, 
and the ‘ labyrinthic’ structure is admirably displayed (Plate LXXYL). The reason of 
this difference is probably to be found in the composition of the Sand of which these 
fabrics were constructed (§7). 
4. In a third set of specimens the solidification is partial only, the infiltration having 
extended from without inwards through a greater or smaller number of concentric 
layers ; and in these the original Arenaceous framework of the unconsolidated interior 
has for the most part disappeared, so that the spheres, when laid open, are found to be 
hollow, the central cavity being occupied only by loose sand-grains, or being altogether 
void. In such specimens the consolidated portion is usually very dark and opaque, so 
that sections can with difficulty be rendered thin enough for being viewed by trans- 
mitted light. — It is worth notice that in one of these specimens of which the outer layers 
have been completely consolidated, these are succeeded by partly unconsolidated lamellae, 
and these again by another set of lamellae whose consolidation is complete ; and this 
alternation repe'ats itself twice as we pass inwards to the central vacuity, which in this 
instance has a diameter only about one-fourth that of the entire sphere. 
5. The diameter of the largest Parkeria which I have seen (this being a specimen 
from the Isle of Wight, in the British Museum) is exactly 2 inches ; that of the smallest 
(the silicified specimen in the Museum of Practical Geology) is *75 inch. Other speci- 
mens range between these two extremes ; the average diameter of those I have seen 
being somewhat less than an inch. 
6. The External Surface of the silicified specimen is covered with small rounded ele- 
vations separated by intervening depressions, so as closely to resemble that of a mul- 
berry (Plate LXXII.) ; and I am disposed to believe that this is the normal condition of 
the fabric, — the dissimilar kinds of surface presented by other individuals being probably 
the result of abrasion. Thus in one of the specimens in my possession, which has not 
been solidified by infiltration, the summits of the tubercles have been worn off by 
abrasion, so as to lay open their labyrinthic structure, the surface of the depressed por- 
tions remaining unbroken ; whilst in another, which has undergone infiltration, the sur- 
face is everywhere uniform, as if the tubercles had been rubbed down to the level of 
the intervening depressions. In some other cases, however, a condition of surface pre- 
cisely the reverse of the first is observable, the rounded elevations being replaced by de- 
pressions which are separated by elevated ridges : and this seems to depend upon the 
consolidation of the originally depressed portions by infiltration, whilst the tubercles 
and the areolae from which they rose continued uninfiltrated ; so that abrasion would 
have the effect of wearing down the latter (as the more friable substance) even below 
