THE 



GEOLOGICAL MAGAZINE. 



NEW SERIES. DECADE III. VOL. I. 



No. I.— JANUARY, 1884. 



I. — A Faulted Slate. 



By J. J. Harris Teall, M.A., F.G.S. 



(PLATE T.) 



IT is well known that the slates of the Borrowclale series in the 

 Lake District furnish beautiful illustrations of faulting on a 

 small scale ; but so far as I am aware, no description of them, from 

 this point of view, has as yet appeared. The accompanying Plate 

 has been produced in Autotype from one of these slates, the surface 

 of which was first most carefully smoothed and afterwards varnished. 

 Every detail of the faulting is shown in the most pei'fect manner, 

 and the general tint of the slate is also reproduced. The specimen 

 was purchased at the village of Eosthwaite in Borrowdale ; but 

 I was not able to learn the exact locality from which it was 

 obtained. I have little doubt, however, that it came from the 

 Honister quarries. Mr. De Eance tells me that similar slates occur 

 at Tilberthwaite, near Coniston. The plate is of the natural size. 

 The face represented is a cleavage plane, and neither the bedding nor 

 the fault planes are at right angles to this face. The bedding planes 

 make with it an angle of about 40°. 



Looking at the plate, the first point that strikes one is the con^ 

 spicuous fault which slopes downwards from right to left. This will 

 be referred to as the main fault. It produces a displacement of S|- 

 inches measured along the line of fault as this is shown on the 

 plate. Eunning parallel with this main fault are several smaller 

 ones, the most conspicuous of which occur in the bottom right-hand 

 corner of the plate. All these form one series which will be 

 described as the B series. Crossing this series at an angle of 35°, as 

 measured in the plane of the face, is another series which will be 

 referred to as the A series. Owing to the intersection of the two 

 series, a number of typical trough faults are produced, and it will 

 therefore be interesting to examine the different modes of explanation 

 that have been proposed to account for such faults. 



According to Prof. Jukes (Manual of Geology, p. 215), trough 

 faults are produced during the bulging upwards of a mass of strata 

 by the action of an upheaving force. Tension must arise in such a 

 bending mass and intersecting cracks may be produced. As the 

 elevation proceeds, the wedge-shape masses may slip down into the 

 opening cracks, and a series of trough faults may arise. When the 

 force of elevation has expended itself, a settling down of the mass 



DECADE III. — VOL. I. — NO. I. 1 



