E. B. Medlieott — On Faults in Strata. 481 



helped to the coincidences exhibited by the figures in those Tables ; 

 but on the whole they fairly support the interpretations put upon them. 



The abstract statement of the theory is very intelligible : when a 

 force acts transversely upon a stratified mass, it tends (by incipient 

 folding or by cleavage) to produce fissures at right-angles to its 

 direction; this is the system which (when discernible) would be 

 called Primary, When a force again acts upon the mass thus fis- 

 sured its elBE'ect will vary: if the component perpendicular to the 

 Primary fissures predominates (a continuation of the first force 

 would be altogether so), the first tendency will be to form cracks at 

 right-angles to the first set, and to be called the Primary-Conjugate 

 system. A continuance of this force, or a force having a predomi- 

 nant component parallel to the Primary fissures, will tend to produce 

 fissures having angles with those systems, respectively dependent 

 upon the coefficient of friction of the rock ; thus forming what Dr. 

 Haughton calls Secondary systems, oblique to the direction of the 

 force ; and which are in turn conjugate to each other. 



Upon applying the known coefficients of friction of various rocks 

 to the construction just stated, the range of variation of the angles 

 is found to agree with that derived from observations between the 

 several conjugate systems. Thus Professor Haughton has achieved 

 a demonstration that rocks in undergoing fracture in the earth's 

 crust are subjected to mechanical laws. " From this it follows that 

 a single hypothesis as to the direction of a system of forces is suffi- 

 cient to account for the existence of these conjugate systems of joints, 

 involving six directions, and it alsa explains the rudely hexagonal 

 jointing of many rocks, if the coefficient of friction be such as to 

 render the angle 6 nearly 30°." It is not clear to me how the 

 original force, normal to A, can produce the conjugates to its two 

 Secondary fissure^planes ; but let that pass.^ 



In thus introducing an intelligible principle of order into a chaotic 

 subj.ect. Prof. Haughton has conferred a great boon upon field-geolo- 

 gists ; but I fail to perceive the independent practical importance the 

 author would attach to it. He concludes with this remark: — 

 " Many other consequences flow from the preceding investigations, 

 into which I have not time to enter, but which may be readily found 

 and turned to practical use by the field- geologist who prefers one 

 hypothesis to many." While paying field-geologists the compliment 

 of supposing them as clear-sighted as himself. Prof. Haughton rather 

 passes a slur upon their pursuit, by suggesting that their judgments 

 are for the most part an arbitrary decision between hypotheses. The 

 notion indicates appropriately the geological barrenness of the 

 theory. These systems, manipulate them how we may, give one a 

 very minimum of information upon the rocks. Because a conjugated 

 system of fracture may have been formed by one and the same force, 

 it does not at all follow in any particular case that they were so 



1 The possibility of widely discordant systems of fissures being produced by one and 

 tbe same act of d&sturbance, as proved by Professor Haughton, seems completely to 

 stultify such attempts as that of M. Moissenet to identify the fissures of a single 

 locality with many distinct systems of the Roseau Pentagonal. 



