112 Major-Gen. McMahon — Culm-measures at Bude, Cornwall 



also filled with crystalline quartz, which is traversed by a third 

 crack — the youngest in order of birth — stopped with carbonate of 

 lime. The quartz in these cracks is full of liquid cavities with 

 moving bubbles, but these inclusions do not orient in lines having 

 a common direction. The feature to which I desire to call especial 

 attention is that the second vein, in order of birth, has its quartz 

 split up into narrow ribbon-shaped crystals, approximately at right 

 angles to the direction of its length, but the older vein is totally 

 devoid of this structure, and the carbonate of lime that stops the 

 youngest vein exhibits no twinning, The twinning of calcite is 

 commonly held to be the result of pressure, and I think the division 

 of the second vein throughout its length into stripy quartz crystals, 

 narrow as compared with their length, must be due to strain. We 

 have then three cracks formed and filled with crystalline materials 

 at different periods. No. 2 cuts the first formed at an average angle 

 of 35°, and the last formed vein cuts No. 2 at an angle of 16°. The 

 mineral stopping No. 2 exhibits evidence of strain, whilst the 

 minerals stopping Nos. 1 and 3 are devoid of anj' such evidence. 

 These facts seem to me to indicate that the strain which has left its 

 marks on No. 2 could not have been applied subsequent to the 

 crystallization of the mineral contents of all three veins, for had 

 it been, there seems no reason why Nos. 1 and 3 should not have 

 exhibited the marks of strain as well as No. 2. The facts seem to 

 lead naturally to the inference that the strain which has left its 

 marks on No. 2 was applied at a critical stage in its history when 

 the quartz was in a condition to yield to that strain ; and as this 

 strain was applied after the quartz in No. 1 had passed that critical 

 stage, and had hardened into its present condition, it was unaffected 

 by it. 



The evidence afforded by these veins teaches us that we should 

 hesitate before we accept the evidence of strain exhibited by 

 individual minerals, or groups of minerals, as proof that this strain 

 was applied after the consolidation of the whole rock. This is an 

 important point which is sometimes lost sight Qf. 



Conclusion. — The Culm series, as seen at Bude, appear to have 

 been deposited in tranquil water undisturbed by strong currents. 

 Though there is occasional evidence of current, this is exceptional ; 

 and the materials of which the rocks are built up seem, on the 

 whole, to have settled down quietly without much sorting, and 

 without the longer axes of the grains being arranged in a common 

 direction, which would have been the case had there been a sensible 

 current. The layers of carbonaceous matter of organic origin point 

 to the same conclusion. 



Mr. Medlicott, in his Memoir on the Sub-Himalayan Rocks of 

 N.W. India, 1 quotes the following remarks of a colleague, Dr. Kane, 

 on the fossil leaves from the Tertiary rocks . of the Sabathu group 

 collected by the author of the memoir : — " A number of well- 

 preserved plant-remains were found in the rocks of the Kasauli 

 range. They are probably of Middle Tertiary age, and are 

 1 Memoirs G.S.I, vol. iii. p. 97. 



