150 



From Professor Sedgztnck. 



Trinity College, Cambridge, 

 October 29tb, 1836. 

 My dear Sir, — 



I am just returned from a geological tour of nearly four months in 

 Devonshire, Cornwall, and Pembrokeshire ; and I have now brought to a close a 

 series of observations on the older strata of England, which I began in 1822. When 

 I shall be able to get my observations down on paper I hardly know ; as new matter 

 is continually rising up, and my health when I lead a sedentary life, is never very 

 good. When I began my work in Westmoreland, (the year before I became 

 acquainted with you at Keswick) I was puzzled to death at first by the appearance 

 of three parallel systems of planes, traversing many of the coarse upper slates. 

 These were joints, cleavage planes, and beds. But which were the true beds? I 

 found them to be indicated by the stripe and other appearances not easy to describe. 

 I also find among my old memoranda a notice of two sets of master-joints : one set 

 I call dip joints, because they are nearly transverse to the beds, and therefore 

 coincident nearly with the dip — another set, strike joints, nearly at right angles to 

 the former, and therefore nearly in the direction or strike of the beds. Unfortunately 

 during subsequent years, though I often remarked the joints and the rhombohedral 

 soUds into which they often divided the slate rocks, I never noted their direction by 

 the compass. Now during the past summer, I have found that in the Granite from 

 Dartmoor to Land's End, there are three sets of master-joints. First set— horizontal, 

 or nearly so, on the tops of the moors ; Second set — nearly vertical, and between 

 Mag. N. and True N. ; Third set — not far from Mag. E. and W. These divide 

 the granite into great cuboidal masses. Besides these master-joints, there are of 

 course many "cracks, backs, slides, flaws," etc., etc., which are quite irregular and 

 undefinable. The Granite workers split the blocks parallel to the three joints above 

 mentioned, and the forces required to split the blocks in these directions are nearly 

 as the Nos. one, two, and three, respectively. Again, the longer axes of the felspar 

 crystals are on the average nearly parallel to the N. and S. joints, it is therefore 

 quite certain that these joints are (at least in part,) due to crystalline action. In 

 these observations, I have only verified what others have done before ; I have also 

 observed in many places, that the joints of the granite 2Jass, ivithout interruption, 

 through the contiguous slates. Such slates are however crystalline, and altered by the 

 contact of the granite, or probably became solid nearly at the same moment with it. 

 I have however, observed this Summer, traces of parallel systems of Mag. E. aud 

 W., and Mag. N. aud S. joints in calcareous slates very far from the granite, and 

 many miles beyond its influence. PhilHps, in his geology of Yorkshire, has found 

 the master-joints in the chain of mountain limestone affecting the above-named 



