20 Pengelly — Devonian Bocks of Torhay. 



most distinctly in slates — to split into illimitably tMn laminEe, 

 whose planes are more or less inclined to those of stratification. 



Jointage is the name given to divisional planes which occur in 

 both stratified and unstratified rocks, follow definite and constant 

 directions through great tracks of country, and by their intersec- 

 tions resolve the rocks into blocks having constant forms. 



The spaces between the joints are very variable, but, in virtue 

 of the jointage planes, the rock is never capable of being split into 

 illimitably thin plates. 



The following are amongst the questions on which the geologist 

 desires information respecting the rock -joints of any district. Their 

 origin, their directions, and their ages, both in relation to one 

 another, and to the chronological periods of geological science. 



That they have been superinduced, or formed since the rock 

 itself was formed, there can be no doubt ; but it may be questioned 

 whether any satisfactory explanation of their origin has been given 

 to the world. There seems a general tendency to ascribe them 

 chiefly to contractions in the mass of the rock during its consoli- 

 dation. It has also been suggested that in some cases they may 

 have been formed in a mass of rock that is in a state of tension 

 from a mechanically expanding force. Professor Harkness supposes 

 that in many instances they may be due to pressure ; whilst others, 

 in their despair, have invoked the aid of electricity, or some other 

 polar force. 



As long ago as 1839, it was remarked by Sir H. De la Beche 

 that "As a whole the great divisional planes of Devon and Cornwall 

 may be said to prevail more in directions from N. to N.W. and from 

 S. to S.E. than in others, the greatest number holding their courses 

 within a few degrees of N.N.W. and S.S.E. ; these lines being cut 

 by others, which chiefly form angles from 70° to SO*^ with them," 

 (Eeport, page 274.) 



It has long been known also that the joints which affect a more 

 or less northerly and southerly direction always cut through, and 

 are therefore more modem than, those which bave a course approxi- 

 mately east and west ; but so far as I am aware no definite informa- 

 tion has been given beyond this on the chronology of joints. I am 

 not without a hope that trustworthy facts will be produced in this 

 brief communication by which the exact geological age of the north 

 and south joints of Devonshire and Cornwall may be established. 



The Devonian slates and limestones in the Torbay district are 

 traversed by two well-defined systems of joints ; one having a direc- 

 tion from N. 82° E. to S. 82° W., the other very nearly in the 

 present magnetic meridian or about N. 23° W. to S. 23° E., so that 

 the two systems intersect at nearly right angles. As a matter of 

 convenience I shall in this communication speak of them as the east 

 and west and north and south systems respectively. The first has 

 commonly a considerable underlie towards the south, whilst the 

 second system is in most cases sensibly vertical. 



It is not my intention to enter on a consideration of all the 

 phenomena and relations of these joints, but simply to call atten- 



