in Labrador Felspar. 323 



pose, in a series of papers, to submit the results to the Royal So- 

 ciety. 



In my account of the Cavities in Topaz, and other minerals, I 

 have mentioned the frequent occurrence of strata of cavities, so 

 minute that they are scarcely capable of being resolved by the 

 most powerful microscope. In the larger cavities, their depth is 

 sometimes very small, compared with their other dimensions ; 

 but in the more minute pores, as they may be called, there is 

 a greater equality in their length, breadth, and thickness, and I 

 have never been able to recognise any thing like the colours of 

 thin plates reflected from the strata which they compose. 



In seeking for the new fluids in Labrador felspar, the fine 

 changeable tints of that mineral could not fail to excite particu- 

 lar attention ; and after examining some specimens, I discovered 

 a new set of colours, which seemed to be capable of a distinct 

 analysis. When these colours are seen by a microscope, and un- 

 der strong illumination, they form a highly beautiful phenome- 

 non, somewhat resembling Fig. 1. Plate XIII. 



The coloured portions have the form of parallelograms, some- 

 times complete, sometimes truncated at the angles, and some- 

 times so rounded as to have no regular outline. Their longest 

 sides are generally parallel to one another, and they are some- 

 times arranged in groups, with their homologous lines in diffe- 

 rent directions. The parallelograms are not distributed in a 

 single stratum. They appear at different depths ; and those 

 which are much below the surface have little brilliancy, owing 

 to the imperfect transparency of the mineral. These coloured 

 spaces vary from the 40th or 50th of an inch in length, to the 

 most minute point which the microscope can descry. 



The tints reflected from these spaces are generally very bril- 

 liant. They are sometimes white, and sometimes all of one co- 

 lour, but I have never found them below the blue of the se- 

 cond order of NEWTON'S scale. The surface which reflects them, 



VOL. XI. PART II. S S 



