108 Major- Gen. McMahon — Culm-measures at Bade, Cornwall. 



anything like an adequate impression of the severity and extent 

 of the contortion they have undergone. 



In the accompanying illustration I have attempted to give a general 

 idea of one of the least complicated sections. Imperfect though my 

 sketch undoubtedly is, it will, I trust, suffice to show that the rocks 

 have sustained a pretty good squeeze. The black portion (g g') 

 represents a bed of dark carboniferous shale ; (a, b) is evidently a 

 fault or sliding-plane, the beds below (a, b) being crumpled in the 

 manner represented in the sketch; (c, d) represents the jutting 

 edge of the cliff, the portion (e, f) stands back from it at some dis- 

 tance, and the connection between the convolutions of the strata at 

 (e, f) and the beds between (a, c, d) cannot be seen from the point 

 of view from which the sketch was taken. 



A few miles further to the south, at Millhook, there is another 

 interesting cliff, known locally as the Zigzag rock. Here the rocks 

 were first compressed into an elaborate series of sharp V-shaped 

 flexures, and then canted over en masse into a vertical position, so 

 as to favour the impression that the squeeze came from above. A 

 photograph of those rocks has been published in the Frith Series 

 (No. 13144). It is an interesting photograph, but the stratigraphical 

 details are not brought out with the clearness desirable for geological 

 study. 



As the Culm-measures at Bude have incontestably suffered from 

 intense mechanical compression, I carefully collected good samples 

 of these i - ocks in the field, and brought them back with me to 

 London for study under the microscope. I was anxious to see 

 whether they threw any light on the problem of dj'namic meta- 

 morphism, and I was therefore especially careful not only to collect 

 samples from all the principal varieties of rocks, but to take my 

 specimens from places where the rocks had suffered most strain. 

 For instance, if a bed had been doubled up on itself, I took my 

 specimen from the knee of the bend ; or if the fold had resulted 

 in the snapping of the bed, from the point of rupture, where the 

 strain had presumably been greatest. Of the darkest and most 

 carbonaceous beds I was unable to obtain specimens, as they have 

 been, under the influence of pressure, smashed up into a wafery dust, 

 possessing no cohesion. A reference to the illustration will show 

 how these beds are squeezed thin in some places ; are pinched off 

 in others ; and swell out to bulky masses in yet other places. Here, 

 at least, if nowhere else, I hoped to find the long-sought evidence 

 of the molecular activity and of the chemical and mineralogical 

 changes induced by great pressure. All that I found, however, was 

 a dust of wafers, in which you might search in vain for a fragment 

 as thick or as large as a sixpence. Pressure had failed to produce 

 even consolidation or induration. 



Since my return to London, I have examined under the microscope 

 thin slices taken from the hand-specimens of the other beds, and I 

 now proceed to state the results! 



Macroscopically considered, these specimens are very fine-grained 

 rocks, and vary in colour from a pale yellowish grey through pale 



