﻿196 DE. H. C. SORBT 02s" THE APPLICATION OF [^^y I908, 



deposition varied from, about 7 inches to 3 feet per hour. A second 

 specimen from another locality indicates a rate of from 9 inches 

 to 3 feet per hour. Different specimens of the slate-rocks near 

 Moffat indicate deposition of from 12 to 18 inches per hour. 

 Yarious specimens from near Bangor indicate from 9 to 18 inches 

 per hour. On the ^hole, my specimens give from 7 inches to 

 3 feet, but the usual rate is from 9 to 18 inches per hour. These 

 may seem quick rates, but of course they refer to periods of decided 

 current bringing deposit, which may have been separated by long 

 intervals without deposition, as in the case of the green slates 

 described later. 



I have made thin sections of fine-grained rocks which break up 

 at once when wetted, by hardening them with Canada balsamj; 

 but, until quite recently, I never suspected that anything of special 

 interest could be learned from sections of soft clays, and it is now 

 quite out of my power to collect and prepare such material. 

 Judging from my experiments, much could be learned respecting 

 the conditions under which such rocks were formed. It would be 

 a wide branch of study, and would necessitate microscopical work 

 on a large scale. The examination of the rocks in a natural 

 condition is enough to show that the structure of clays differs 

 enormously, and indicates formation under very different conditions ; 

 but there is always some doubt as to their true structure, when 

 not made into thin sections. 



A puzzling question is the origin of thick beds of almost homo- 

 geneous fine grain, like the slates of Penrhyn and Llanberis, since 

 ^ney may have been formed in two different ways, either by a 

 gentle and uniform current, continuously drifting very uniform 

 material, or by the deposit of cohered mud from very tranquil 

 water. It is, however, possible that the above-named slates may 

 originally have had a thin laminar structure, which was obliterated 

 when cleavage was developed. So far as can be judged from the 

 rocks in a natural condition, a homogeneous structure is common 

 in oui^ later deposits, and seems to indicate drifting by a very 

 gentle current to spots where there was scarcely any at all. As 

 examples, I may name deposits in old lakes, much of the Gault and 

 Speeton Clay, and some Coal-Measure shales. The structure of 

 much of the Boulder-Clay must be explained in a different manner. 

 The Gault of Aylesford, the Kimmeridge Clay near Piley, and the 

 Lias near Whitby, show laminar structure due to currents. 



We thus see that the history of fine-grained rock is written 

 in a well-defined special language, still imperfectly understood, 

 for want of adequate experimental study, both in the field and 

 microscopically. 



X. The GpvEex Slates of Lax&dale. (Pis. XIV-XYI.) 



Probably no better example could be found of the effects of 

 currents acting for a short time, than specimens that I procured 

 from the quarries in Langdale. These structures would probably 



