720 
Proceedings of the Royal Irish Academy. 
separated, out of an original mass of igneous matter, all the different 
constituents, and placed each as they are now found in regular order 
for miles. Shearing in combination with its adjuncts might have 
separated the constituents ; but if so, they would occur in spots, but 
not in continuous beds miles in length. On the other hand, if the 
argillaceous, calcareous, and arenaceous matter were successively accu- 
mulated, and afterwards metamorphosed, we should find successive 
strata like those in this series of beds. 
The similarity of the order in the different sections is the more 
conclusive when we take into consideration that the continuation of 
the original beds has received rough treatment, they having been 
broken and shoved about by heaves, upthrusts, and other faults. 
The present structures in the metamorphic limestones seem also to 
suggest to me that originally they were oi dinary accumulations.^ In 
many limestones there are both cherty and shaly layers, while 
disseminated in all limestones there are impurities in a greater or less 
degree. These impurities, when the rock is metamorphosed, must be 
concentrated along the lines of foliation. The structures in limestones 
have characters of their own, evidently due to chemical and molecular 
changes that began from the time the rocks began to accumulate. This 
is evident if we study artificial limestone, such as the plaster on a wall, 
or a dead barrel or sack of cement that has been allowed to spoil and 
become stone. To the structure in old plaster my attention was first 
called by Mr. A. B. Wynne, who pointed out that when it was exposed 
to atmospheric influences, and had weathered, it had characters similar 
to limestone. In it were lines like bedding, indicating the top and 
bottom of each course put on by the plasterer, a spheroidal structure 
due to the plasterer working his implement round and round, opposite 
to where he stood, while to the right and left there were inverted 
curves in different directions, due to pushing the implement in opposite 
ways, and then drawing it back again. These different motions must 
have more or less separated, imperceptibly, the constituents of the 
mortar, the lines of separation afterwards being better developed by 
molecular and chemical changes . 
A study of dead cement, whether in barrels or sacks, if followed 
from the filling to the time it spoiled, is very instructive. The barrels 
or sacks may have been filled with the shovel, or by a continuous sup- 
ply from a shoot. But in either case, after each is filled, it is shaken 
^ I do not exactly conceive how, on the shearing hypothesis, one wonld account 
for limestone beds in the torn-up eruptive rocks. 
