TRANSACTIONS OF THE SECTIONS. 77 



passage-beds between the two. Mr. Reid pointed out that the Limestone and 

 underlying Devonian Slates were faulted against the Culm-measures on the west, as 

 supposed by Jukes ; and he had been able to trace the fault through certain points 

 which had previously been held as evidence of unconformability. 



On a new Method for Studying the Optical CJiaracters of Minerals. 

 By H. C. Sokby, F.R.S. 4'c. 



The author first described the principles on which this method depended, and 

 showed that the great difference between the appearance seen with the naked eye 

 and the microscope is due to the object-glass being able to collect divergent rays. 

 In looking with a two-magnifying power at a small circular hole seen through a 

 section of a crystal, very different phenomena present themselves, according to its 

 optical characters. If it has no double refraction, only one well-defined circular 

 bole can be seen. If the mineral possess double refraction and only one optic axis, 

 like calcite, two images of the hole are seen. If the section be cut perpendicular to 

 the_ axis, two circular holes are seen directly superimposed but at two different 

 foci. If the section be in the plane of cleavage two widely divided images are 

 visible— the one due to the ordinary ray being circular, and the other, due to the 

 extraordinary ray, being distorted and drawn out in two opposite planes at two 

 different foci. When the section is cut parallel to the axis, this image due to the 

 extraordinary ray is still more elongated, but the images are directly superimposed. 

 We thus at once learn that the mineral has double refraction, has an optic axis, and 

 also what is the direction in which the section is cut. In the case of crystals like 

 aragonite, which have two optic axes, there is no ordinary ray ; and at two focal 

 points we see tbe circular hole drawn out in opposite planes into crosses. The 

 character of these crosses depends upon the direction of the section ; but the fact of 

 the crosses being seen at once proves that the mineral has two optic axes. Some 

 facts are better observed if, instead of a circular hole, we examine through the 

 crystalline plate a grating with two systems of lines at right angles to one another. 

 We then obtain what the author calls unifocal or bifocal images, according to the 

 systems of crystallization. Crystals without double refraction have only one unifocal 

 image ; crystals having one optic axis have one unifocal and one bifocal image ; 

 whereas crystals having two optic axes give two bifocal images. The definition of 

 unifocal images is independent of the position of the lines ; whereas in the case of 

 bifocal images the lines are distinctly visible only when they are parallel or perpen- 

 dicular to a particular axis of the crystal, and spread out and become obscure and 

 disappear when rotated to a different azimuth. The above-named general characters 

 differ so much in different minerals that they furnish a most valuable means for 

 their identification. 



Influence of the Position of Land and Sea upon a Shifting of the Axis of the 

 Earth. By Arthur War. Waters, F.G.S., F.L.S. 



The author pointed out how the unequal distribution of land and sea may be an 

 agent in preventing the movements of elevation and depression of the land, in one 

 part of the globe, balancing those in another, and further showed how similar 

 movements in various localities woidd differently affect the pole. 



Any movement, such as a submarine elevation, which displaced water would 

 spread it over the oceanic area ; and the effect of this woidd, with the present con- 

 figuration, be the same as if about -jV of the weight had been added in the southern 

 hemisphere along 45° 44' long. E., viz. in a line passing by the entrance of the White 

 Sea over the Caucasus through the middle of Madagascar. 



As every submarine movement woidd create a force acting in this direction, there 

 seems reasonable grounds for thinking the tendency woidd be for the shifting of the 

 axis to take place near this fine. Dr. Jules Caret considers that the pole must have 

 moved approximately in a line passing through 52° long. E., and what is cause/what 

 effect, and how far they react on one another is fully worthy of examination by any 

 physical geologist. 



