64 
ELEMENTS OF GEOLOGY. 
abounding in the magnesian limestone of the north of Eng¬ 
land. The spherical balls are of various sizes, from that of a 
pea to a diameter of several feet, and they have both a con¬ 
centric and radiated structure, while at the same time the 
laminae of original deposition pass uninterruptedly through 
them. In some cliffs this limestone resembles a great irreg¬ 
ular pile of cannon-balls. Some of the globular masses have 
their centre in one stratum, while a portion of their exterior 
passes through to the stratum 
above or below. Thus the larger 
spheroid in the annexed section 
(Fig. 49) passes from the stratum 
b upward into a. In this instance 
we must suppose the deposition 
Spheroidal concretions in magnesian ^ series of minor layers, first 
lines one. forming the stratum and after¬ 
wards the incumbent stratum a; then a movement of the 
particles took place, and the carbonates of lime and magne¬ 
sia separated from the more impure and mixed matter form¬ 
ing the still unconsolidated parts of the stratum. Crystal- 
ization, beginning at the centre, must have gone on forming 
concentric coats around the original nucleus Avithout inter¬ 
fering with the laminated structure of the rock. 
When the particles of rocks have been thus re-arranged 
by chemical forces, it is sometimes difficult or impossible to 
ascertain whether certain lines of division are due to original 
deposition or to the subsequent 
aggregation of similar particles. 
Thus suppose three strata of grit, 
A, B, C, are charged unequally 
Avith calcareous matter, and that 
B is the most calcareous. If con¬ 
solidation takes place in B, the concretionary action may 
spread upward into a part of A, where the carbonate of lime 
is more al)undant than in the rest; so that a mass, d e 
forming a portion of the superior stratum, becomes united 
Avith B into one solid mass of stone. The original line of 
division, d 6, being thus effaced, the line d would generally 
be considered as the surface of the bed B, though not strictly 
a true plane of stratification. 
Pressure and Heat. —When sand and mud sink to the bot¬ 
tom of a deep sea, the particles are not pressed down by the 
enormous weight of the incumbent ocean; for the water, 
which becomes mingled with the sand and mud, resists press¬ 
ure Avith a force equal to that of the column of fluid aboA^e. 
The same happens in regard to organic remains which are 
Fig. 50. 
A 
' I’T 
■'iIbIV'i 
C 
