On the MetamorpJiic Rocks of Counties Sligo and Leitrim. 363 



Some specimens now exhibited have this property in a very 

 marked way. 



It is towards this side also that the fibrous mineral most pre- 

 dominates, and this mineral also contains a very large quantity of 

 the magnetite, which is disposed in lines along the planes of 

 lamination, and, as in the case of the other portions of the rock, 

 almost parallel to the magnetic meridian. 



Polarity of the Roch. — As to the precise direction of the poles 

 of the rock, I am not quite confident, but I believe them to 

 lie for the most part very nearly north and south — the N. pole of 

 course to north. There is a difficulty in obtaining specimens out 

 of the mass, in such a way as certainly to identify their original 

 position, the rock being very splintery ; but the majority of my 

 observations leads me to the above opinion. In at least one case, 

 however, the poles were inclined with the dip of the rock. The 

 north pole being lawest, and towards the west, the south being 

 uppermost. 



Effects on Surveying Instruments. — As may be supposed, the 

 rock has a very decided efifect on the magnetic needle, and I 

 made some few experiments to test the amount of this. Taking 

 a point about the centre of the rock, I made a traverse on each 

 side for about 300 yards, and I found a difference of 5° in the 

 readings of the compass between the points on each side. It 

 was, however, very difficult to be certain as to the North point, 

 and I did not altogether rely on these observations. In order to 

 make sure, the following plan was adopted : — A tree, nearly due 

 west of one on the rock was chosen, and about 1000 feet distant. 

 The observation with the prismatic compass was as follows : — 



Eeading from distant tree to that on rock - 90° 80' 



„ „ tree on rock to above - - 264° 00' 



The correct reading should be - - - 269° 80' 



26 i° 00' 



Therefore disturbance of needle - = 5° 30 to E 



The direction of a portion of the road was 



taken (close to rock) - - = 177° 00 



Direction as obtained from 6-inch map = 175° 00 



Deflection - _ _ 2" 00. 



The prismatic compass I used was the reverse of delicate, and not 

 much affected by small portions of the most magnetic specimens. 



