MICKOSCOPIC STRUCTURE OP FELSPARS. 485 



not think petrologists have hitherto given much consideration. If 

 a section be cut obliquely to the twinning-planes, and if the alternate 

 planes polarize in complementary colours, there may be (as indicated 

 in fig. 10, PI. XXIY.) an overlap of the different lamellae, which 

 would result in the recompounding of the coloured light, and thus 

 give rise to bands of ordinary white light, which under crossed Nicols 

 would appear dark ; and these bands would occupy the areas lying 

 between the upper and lower sections of the obliquely situated 

 twinning-planes (w, w, w, fig 10). Supposing the section to be 

 cut still more obliquely to these planes, as in fig. 11, PI. XXIV., the 

 overlaps would occur in a continuous series, so that although the entire 

 section might consist of numerous twin lamellae, there would be no 

 coloured polarization to evince their presence. Pig. 8, PI. XXIY.. 

 shows two dark bands (under crossed JSTicols) bordering a twin la- 

 mella, in a plagioclase crystal from a section of basalt from the 

 Cleveland District, given me by my friend Dr. Pritzgartner. Pig. 7, 

 PI. XXIY. is a very curious little crystal occurring in the same sec- 

 tion of Cleveland basalt. The elliptical area surrounds a small dark 

 speck ; and although the crystal is twinned, the boundary line of 

 this internal area does not appear to have been affected, and it may, 

 I think, have been produced subsequently to the development of the 

 twin. A small band is visible in one of the halves of this twin, also 

 running parallel to the longer axis of the crystal. This appears to 

 be one of those difficult examples in which the observer cannot 

 help feeling doubt as to the system to which to refer the felspar. 

 The feeble stripe just alluded to, and its occurrence in a basalt, are 

 the only evidence to support its claim to be regarded as plagioclase. 

 The occurrence of orthoclase in basalts has already been pointed out 

 by Mr. Allport, in his paper on the Carboniferous Dolerites read 

 before this Society last year ; and although most observers regard it 

 as a doubtful question, and Zirkel considers such crystals, if they do 

 so occur, to be merely sporadic, still I am inclined to think that 

 Mr. Allport's conclusions on this subject are not far from the truth, 

 and that orthoclastic crystals are of more frequent occurrence in 

 some dolerites than has hitherto been suspected. I do not, however, 

 believe that they could occur to any great extent : if they did, the 

 character of the rock would be completely changed ; it would no 

 longer be dolerite. 



In all these matters, I believe that the micro-petrologist does well 

 to be guided by prevailing opinions ; but if progress is to be made, 

 the truth of those opinions should be tested rigidly ; and the demon- 

 stration of small facts may then enable him to grapple with larger 

 questions. Although we may be able to deal speculatively with 

 large questions affecting rock masses and their origin, still, in the 

 absence of demonstration, doubt will always exist, and we can never 

 be the masters of those questions until we understand the details of 

 which they are built up. 



Supposing the foregoing statements of the means at present at 

 our disposal for the microscopic determination of monoclinic and 

 triclinic felspars to be correctly given and to represent the truth, 



