163 



tion of a pile of rock, equal in thickness to nearly ^ve miles, in the 

 interval between the close of one formation and the commencement 

 of the next members of the geological series we have any local 

 record of — realising that there is nearly five miles of rock missing 

 between the Silurian rocks of the Lake District and the Carbon- 

 iferous rocks that now skirt their flanks — Harkness's keen spirit of 

 scientific enquiry led him to explore other parts of the kingdom, 

 where some representatives of the enormous lapse of time implied 

 by this "grand unconformity were wholly, or in part, to be found. 

 So we find him setting out for the region that Hugh Miller has 

 rendered classical by the publication of his book on the Old Red 

 Sandstone. But before setting out, besides mastering the literature 

 of the subject, which had then already extended to a considerable 

 length, he is communicating his intention to his fellow geologists, 

 and otherwise preparing himself for making the best possible use 

 of the limited time he was able to devote to the work. The 

 following letter from Sir Charles Lyell well illustrates the nature 

 of the relations subsisting between these two well-known men of 

 science. 



S3, Harley Street, July 5, 1861. 



Dear Harkness, 



I am glad you can go to Forfarshire. I enclose a copy of my section* 

 with the alterations proposed to be made in the woodcut in accordance with 

 your former suggestions. These changes can be made in type by putting in 

 writing and a few lines to make the clay slate dip N.W. 



As to the great anticlinal of the Sidlaw I am well aware of minor inequalities 

 and the intrusion of trap, and perhaps there may be several faults in the line I 

 have chosen, but you will find the dips correct in the main, as I have given 

 them. Indeed, Mr. Powrie says they are so. My object was to simplify to the 

 student. 



But if the beds {a) are conformable, I must modify the section. Perhaps you 

 will be able to suggest how, without disturbing my page, to get over the 

 difficulty. But I will not alter it unless you say that the case of conformability is 

 quite clear. I shall be surprised if there is not an overlapping series. All may 

 be Old Red, but («) may have been formed after the beds 3 and 2 were slightly 

 raised and denuded. 



If I understand Mr. Powrie, he thinks that the conglomerate and sandstone 



* The section illustrating the physical relations of the Upper Old Red to the Lower, 

 inserted in the "Student's Manual." 



