Dr. C. Callaway — Parallel Structure in Rocks. 351 



of the sediment deposited within the limited delta-ai'eas/ remember- 

 ing: — (1), that all stratified rocks are largely formed from the 

 denudation of those previously existing, and (2), that there may 

 have been intervals between successive geological periods represented 

 by formations now unknown to us or not included in the above 

 estimate. 



Putting, then, a = 177,200 feet in the last equation, we find 



T = 23 X 177,200 = 4,075,600 years^ 



i.e. just half what it would have been had the rate of subsidence 

 been uniform and equal to the rate at the present time. 



But, if we may have over-estimated the average thickness of the 

 stratified rocks, we have, on the other hand, taken a superior limit 

 to the rate of subsidence. Also, in the above calculation, it was 

 supposed that sedimentation, like subsidence, began as soon as the 

 earth solidified ; whereas, it is possible that a considerable interval 

 elapsed, thereby postponing the formation of the stratified rocks to 

 a time when the rate of subsidence was slower than at first. 



We may conclude, therefore, on both accounts, that for the formation 

 of the sedimentary rocks a time is required of not less than four 

 million years, and possibly of a period very much longer than this. 



V. — On Parallel Stbuctdek in Eocks as indicating a Sedi- 

 mentary Origin. 

 By Ch. Callaway, D.Sc, M.A., F.G.S. 



IGNEOUS rocks and crystalline schists are often associated with 

 each other in snch a manner as to compel the conclusion that 

 either the igneous rocks have been formed out of the schists or the 

 schists out of the igneous rocks. There are other cases in which the 

 two kinds of rock are less intimately related ; parallel structure is 

 wanting, the junctions are sharply defined, and there is no evidence of 

 an original mineral gradation. These are examples of the ordinary 

 irregular intrusion of igneous rocks in schists. But there is a third 

 group in which the characters are intermediate. In these rocks, 

 structural parallelism is more or less distinct, and there is often a 

 partial blending of the two kinds of rock at the line of contact ; 

 but other indications forbid the belief that the schists have been 

 elaborated out of the associated igneous masses. The question then 

 arises : Are we here dealing with cases of intrusion, or are we 

 driven to conclude that the igneous rocks have resulted from the 

 fusion of schists ? The answer to this question has obviously an 

 important bearing upon the genesis of the crystalline schists. 



The conversion of schists into granite and the like is maintained 

 by the officers of the Geological Survey of Ireland,^ and I have dealt 

 with their evidence in a paper published in the Journal of the 



^ Dr. CroU has very clearly shown that there may be a considerable difference 

 between the maximum and average thicknesses of a formation ; but it will be 

 remembered that he calculated the average over a belt 100 miles broad round the 

 whole coast-line.— Geol. Mag. 1871, Vol. VIII. p. 101. 



2 Survey Memoirs, Nos. 93, 94, 95, 104, 105, 113, 114, passim. 



