AKXIVEESAEY ADDEESS OF THE PEE9IDENT. XXX Vll 



and joints are also reducible to conjugate systems, four in number, 

 two of them nearly agreeing with, the above system of faults. 



In the annexed diagram the more remarkable directions of faults 

 and cleavage -planes appear together : — a^, a^, and ¥, 6^, rectangu- 

 lated fault systems ; c\ c^, &, c*, directions of cleavage-planes. 



From the whole examination it is concluded that the cleavage- 

 planes are perpendicular to the axes of maximum force ; that they 

 were developed while the substance of the rock was soft; and 

 that the joint-planes, which are conjugate to the cleavage -planes, 

 formed by the shrinking of the rock-mass, were subsequent to the 

 cleavage-planes, and formed while the rock was hard. The two 

 systems of conjugate faults are regarded as the result of two distinct 

 systems of upheaving force in the district. 



In all these conclusions Prof. Haughton is much in accordance 

 with what has been delivered on some of the subjects by Mr. Hop- 

 kins, and with phaenomena observed in other districts. I have 

 pointed out one very general similar conjugate system of joints in 

 the I^orth of England, and a rectangular relation of one set of joint- 

 structures to cleavage-planes in North Wales; more than one instance 

 of the coincidence of cleavage-planes and great faults is on record ; 

 but it is now for the first time that we have had these phaenomena 

 carefully examined as a whole, and combined into a general view of 

 the requisite mechanical conditions. 



The general conception of the law of slaty cleavage is completed 

 by Prof. Haughton's investigation of the direction of pressure in a 

 cleaved rock where organic remains have suffered distortion. For 

 the purpose of recording accurate data on this subject fit for complete 

 computation, it is necessary to measure the strike and dip of the 

 strata, the strike and dip of the cleavage, and to determine the 

 amount of distortion which the figure of an organic body has imder- 

 gone on each of two planes of bedding, maldng different angles 

 with the plane of cleavage. It is easy to deduce from these data 

 the amount of change of dimensions which the rock has under- 

 gone. 



By assuming the change of dimension to be only in the direction 

 perpendicular to cleavage, the determination of the distortion on one 

 plane of bedding will be sufficient to determine the amount of 

 compression. 



Tlants. 



The interestuig flora of the Palaeozoic strata has received much 

 attention from the geologists of America, especially Dr. Dawson, 

 who finds in the Devonian strata of Gaspe, besides plants clearly of 

 the family of Lejpidodendra, others, probably also Lycopodiaceous, 

 which he terms Psilophyton. The internal structure shows cellular 

 tissue and obliquely striated vessels ; the branches are somewhat 

 curled at the tips ; the rootlets spread into the argillaceous earth, as 

 the roots of Stigmaria do into fireclay. Mr. Salter informs me that 

 in the Old Eed strata of Caithness he has recognized plants of the 

 same genus. There is another curious plant, which is of the Coni- 



