Position of Strata in Stratified Rocks. 373 



plane is represented, and at the same time it may be made to re- 

 volve round the centre of the instrument, so as to be placed in 

 any required position relatively to any of the points of the com- 

 pass. 



In such a state of things, the half ring will represent the plane 

 of a stratum, its hinge the direction of the stratum, or its com- 

 mon intersection with the horizontal plane. And all the imagi- 

 nary lines which may be drawn from the centre of the instru- 

 ment to the circumference of the ring represent all the possible 

 sections which may be made by vertical planes in the plane of 

 the stratum, and so correspond exactly to the seams or lines of 

 stratification often mentioned above. The divisions on the semi- 

 circular plate under the half ring will represent the horizontal 

 projection of those lines which coincide with the given divisions 

 of the compass. 



Now, by means of a graduated arc, applied perpendicularly 

 to the plane of the semicircle, and touched in one point by the 

 sharpened interior edge of the half ring, we will be able to mea- 

 sure the angle made by any line in the plane of the half ring 

 with its horizontal projection. 



I am indebted to Mr AD IE, member of this Society, for a 

 material improvement, that of substituting a simple half ring, 

 such as I have described, for a semicircular plate traversed by 

 fissures, by which I had at first thought of representing the 

 plane of the stratum. To him also is due the contrivance by 

 which the centre of the divided arc is made to coincide with 

 that of the instrument, and the arc itself to stand vertically, 

 which is absolutely necessary in all cases. For these purposes, a 

 small projecting pin occupies the centre of the instrument, and 

 is received into a small hole corresponding to the centre of the 

 protractor, \\hile a small support fixed at a point of its diameter 

 contributes also in keeping it in a vertical position. 



An instance will now show the mode of using the instrument, 



