WARREN AND PALACHE. — QUINCY PEGMATITES. 149 



The quartz of the massive part of the pegmatites is relatively or wholly 

 free from the minute microlith.s of aegirite and riebeekite so abundant 

 in the feldspar. It appears to have been the last mineral to develop in 

 the normal crystalliiuition of the pegmatite as well as in the granite, 

 and also to have been the last mineral to cease its growth in the sec- 

 ondary deposition which took place in the pocket. 



The Ftkhjxirs. — MicrocUne in well formed crystals of orthoclase 

 habit makes up the greater part of the porous material near the great 

 central pocket. The crystals range from a diameter of two and a 

 half cm. downwards to mere crystiil specks ; they are, however, very 

 constant in habit presenting a remarkably cuboid form due to the 

 dominant development of the base, clinopinacoid, and orthodome ; prism 

 and unit pyramid, the only other forms found, being very subonlinate 

 in size. The faces are smooth and give fairly good reflections of the 

 goniometer signal. The albite twinning, shown by microscopic study 

 to be universally present is not apparent on the exterior of the crys- 

 tals ; its presence makes the crystals sensibly monoclinic, however, 

 and the measurements obtained approximate to those of orthoclase. 

 Well formed Baveno twins are seen in a few specimens but most of 

 the crystals are in clusters without apparent definite relation of the 

 constituent individuals. The color of the microcline is white to pale 

 ivory yellow. On faces of the prism, there is often a secondary 

 coating of colorless glassy feldspar in parallel position to the main 

 crystal which the microscope shows to be also microcline although its 

 appearance strongly suggested the growths of albite so common on 

 orthoclase from numerous localities. 



Orientated sections, cut from the freely developed crystals of the 

 pocket lining and from some of the larger crystals of zone C, show that 

 the micnjcline is twinned after the albite law only and thus lacks the 

 grating structure characteristic of microcline in general. In basal 

 sections the twinning is .seen to be very finely j)olysynthetic. The in- 

 dividual lamellae appear as short strips slightly elongated parallel to 

 010. Their boundaries are as a rule not sharp. The two sets of 

 lamellae extinguish symmetrically on either side of the trace of the 

 twinning plane at an angle of Hi degrees (average of 12 measuromonts). 

 The clearer growths of later age are in parallel position to the older 

 crystal and in them the twinning lamellae are often longer and more 

 sharply defined. In the small microclines throughout the finer grained 

 portions of the j)ipe the twinning is usually more .sharply defined. The 

 extinction in oio .sections was found from the average of ten measure- 

 ments to be ;'» degrees. Figure 17, Plate 3, is u microphotograpli, in 

 polarized light, of a basal section of one of the microcline crystals from 



