directions, viz: — Mag. N. and S., and Mag. E. and W. The same observations 

 have been made by Mr. Hopkins, in Derbyshire, and extended into the coal field. 

 Now, I am anxious that you, whom I respect as a careful and good observer, should 

 bear these facts in mind, whenever you visit the rocks of your neighbourhood. 

 Note the master-joints carefully, and by comparing different points together, try 

 to reduce the results to system. Try also to put Wright on the same scent. The 

 observations should be carefully made and registered : but they are not very 

 troublesome, and may be made from time to time as occasion offers. Among 

 mechanical rocks, I have also seen many examples of joints in the same direction as 

 the dip, and also at right angles to it, i.e. dip-joints and strike -j oints ; as I am in 

 the habit of calling them. These, at least, have nothing to do with the points of 

 the compass. I do however think that I was wrong in what I published some 

 time since, when I stated that joints were due to mechanical, and not to chemical 

 forces. I thought then that they were produced mainly by contraction, when the 

 primeval masses passed into a solid state. This theory may explain the origin of 

 some of them ; but it does not account for others which I now think the results of 

 crystalline action. Besides the stripes, you so well know, parallel to the beds and 

 crossing the cleavage of slates — there are sometimes in Cornwall very fine stripes, 

 resembling scratches along the surface of the Slates, and also a kind of false stripe. 

 The strice and false stripe, are both parallel to joints in the same or neighbouring 

 rocks of the quarries ; I don't remember having seen such appearances in the green 

 slates of Cumberland : but I think there is sometimes a false stripe in the Skiddaw 

 slate. I am not sure I make myself understood ; but I shall be glad to hear from 

 you on these subjects, and when I can procure a frank, I shall perhaps trouble you 

 again with some questions. The winter has already set in with much severity, 

 and we have thick snow on the ground accompanied by hard frost. 



Believe me, dear Sir, 



Very truly yours, 



A. SEDGWICK. 



From Mr. Otlcy. 



Keswick, December 20th, 1836. 

 Sir,— 



I should not have so long delayed acknowledging the receipt of yours of 

 the 29th October, had 1 been able to say anything satisfactory in reply. With 

 regard to the joints in the slate rocks, I have found them running in such various 

 directions that I considered them accidental, and the result of contraction ; and 

 having imbibed that idea, I have observed nothing to make me relinquish it. For 

 some years I have travelled but little, and had few opportunities of conversation 



