Mr HOPKINS, ON RESEARCHES IN PHYSICAL GEOLOGY. 55 



if the mass be stratified, to represent a line of stratification, cd and gh 

 a.\-e faults ; tlie differences of elevation ab, de, fg, are supposed too small 

 to be so designated. Small relative elevations of this kind constitute 

 what is frequently termed the throw of the vein. (Introd. ii. i. p. 4.) 



56. It is important to observe the different effects which will be 

 produced on the form of the longitudinal and transverse fissures by 

 the movements above described. It has been shewn (Art. 38.) that a 

 fissure immediately after its formation, and before any subsequent move- 

 ment of the mass has taken place, must offer a certain approximation 

 to uniformity of width ; but an inspection of the diagram in page 5 1 , 

 will make it appear very evident, that this subsequent movement must 

 in general destroy, in great measure, this character in the longitudinal 

 fissures, since it must almost necessarily close them in some parts and 

 open them considerably in others; while a movement similar to that 

 described in Art. 53, and represented in the figure, page 54, will 

 not necessarily produce any derangement in this respect in a perfectly 

 uniform fissure, because the motion of one wall of the fissure is parallel, 

 or nearly so, to the other. We should expect therefore, as a necessary 

 consequence of this view of the subject, a much nearer approximation 

 to uniformity of width in the transverse, than in the longitudinal fissures. 

 This is strikingly in accordance with what has been stated in the Intro- 

 duction (i. Q. p. 4.) a rule to which, I believe, there are comparatively few 

 exceptions. 



^. Proper signification of the term "System of Fissures" — Simultaneous 

 Formation of Systems of Fissures. 



57. I have hitherto spoken of systems of pai"allel fissures, as if tlie 

 parallelism of the fissures constituted the essential characteristic of each 

 system ; and in the case we have been considering of an elevation of 

 indefinite length, and of wiiich the axis is rectilinear, this parallelism 

 will characterize the two systems at right angles to each other, and which 

 I have designated as longitudinal and transverse. If, however, the axi.s 

 of the general elevation of indefinite length be not in a right line, tlie 

 fissures of the longitudinal system (assuming them to be produced in 



