488 PECULIARITIES IN THE MICROSCOPIC STRUCTURE OE EELSPARS. 



Mr. Forbes considered that, although the so-called twin stripes 

 or strise seen on the cleavage-planes of many felspars were evi- 

 dence of their being triclinic, when regarded as a means for dis- 

 criminating felspars, too much importance is often attached to the 

 striation and cross-hatching seen under the microscope. In an 

 inquiry like this, the first point, he thought, should be to secure for 

 examination and comparison specimens of the different felspars of 

 undoubted purity; and he laid the more stress on this, as a pre- 

 vious acquaintance with the orthoclase felspars from Twedestrand, 

 Arendal, and Ytterby, specimens of which the author now exhibited 

 and described, made him believe them to contain an admixture of 

 oligoclase,' disposed in the same manner as the Perthite of Canada, 

 which is an admixture of orthoclase and albite, a similar arrange- 

 ment' being perceptible even to the naked eye in the specimens 

 exhibited; and he therefore could not regard conclusions drawn 

 from the examination of such specimens as altogether to be de- 

 pended on. 



Mr. Etjtlet briefly responded, pointing out that not merely 

 curved but rectilinear internal boundaries, or divisional markings, 

 occurred in crystals developed in the same fragment of obsidian. 

 He admitted to some extent the justice of Mr. David Forbes's re- 

 marks, but was not prepared to agree with him on the worthless- 

 ness of the microscopic observation of twinning and striation as a 

 rule ; and he considered that if Mr. Forbes's assumption were true, 

 the diagnosis of many eruptive rocks, by ordinary microscopic ex- 

 amination, would become an almost impossible task. 



