LIMESTONE OF THE PORT HENRY LOCALITY. 253 



As the extinction angles range upward to thirty degrees, the plagioclase 

 must be a basic one near the anorthite end of the series, and this 

 might be anticipated from the calcareous surroundings ; but others which 

 range downward to eleven degrees may belong to labradorite. In such 

 variable bodies a quite extended series is undoubtedly present, and the 

 presence of orthoclase is not to be denied. The hornblende is prevailingly 

 dark brown in color, but the green secondary variety is not lacking, and 

 as there are cases where green pyroxene is clearly passing into it, the 

 green may liave been derived from pyroxene in all cases. The pyroxene 

 is a liglit green augite, and is practically the same one that appears in the 

 gabbro and anortliosites. Pvrrhotite is widespread, and biotite is })resent 

 in irregular flakes. The other minerals are best developed, if not almost 

 entirely so, in the more pegmatitic portions, or at least in those which 

 are very coarsely crystalline. The matrix of them is most often quartz, 

 or a very hard mixture of it and hornblende. Coarsely crystalline calcite 

 also occurs. Except the titanite, the crystals are seldom well terminated, 

 although the prismatic ones in this zone may be quite perfect and several 

 inches in length. 



The evidences of slow yielding to pressure are not rare, for even so 

 brittle a mineral as tourmaline is occasionally bent through thirty degrees 

 or more without breaking. It has adapted itself to the " set " and taken 

 the curved shape. Titanites are of the familiar envelope form, with 2 P 

 most {prominent and OP well developed. 



Graphite is very generally mingled with these silicates in quite large 

 plates and scales. Near Port Henry a little group of tiuorite crystals was 

 found, shading from colorless into yellow. The yellow tint is shown by 

 micro-sections to be due to the development of vermicular bodies like the 

 well known " helminths " of chlorite which occasionally occur in quartz, 

 but which in this case are of extraordinary perfection. They possess the 

 greater interest, as fluorite is usually regarded as a mineral which resists 

 alteration. 



While these silicate inclusions in the limestone are at times small and 

 thickly grouped, yet they are found in the quarries of very large size. 

 Individuals -jO feet or more in length and half as much in height are laid 

 bare in several places, and experience shows that no great thickness of 

 limestone is ever devoid of them. 



The Uphicalcites. — As already shown by G. P. Merrill, these are vari- 

 able rocks. At times and for limited stretches they are a homogeneous 

 mingling of small serpentine-pyroxene masses and calcite, but again they 

 have coarse l)lotchy portions which destroy their homogeneity. The 

 writer's obser^'ations corroborate those of Mr Merrill, that the serpentine 

 is in nearly all cases derived from c(jlorless pynjxene, but in several in- 



