118 



DR HATCH ON THE LOWER CARBONIFEROUS 



variety of plagioclase — probably labradorite, for its high extinction angles, measured to 

 the twin-striation, places it near the basic end of the lime-soda series. The larger crystals 

 present the usual phenomena characterising the felspars of volcanic rocks — zonal extinc- 

 tions between crossed nicols, and numerous inclusions (augite, magnetite, &c). The 

 lath-shaped felspars of the ground-mass extinguish in too indefinite a manner to permit 

 of the nature of the felspar being determined. A third stage in the genesis of the felspar 

 is represented by the presence of small patches of a clear unstriped variety. Since there 

 is no glassy base present, these felspar patches apparently resulted from the crystallisa- 

 tion of the mother-liquor of the rock left in consolidation. 



With regard to structure, its most prominent feature is the more or less idiomorphic 

 character of all the constituents. The porphyritic felspars occur in short rectangular 

 prisms ; and the serpentinized olivine, in forms characteristic of that mineral. In the 

 ground-mass the felspars have a long, lath-shaped habit ; olivine occurs in grains which 

 in part are bounded by crystallographic contours ; and magnetite, in rectangular granules. 

 Most pronounced of all, however, is the idiomorphic character of the microlitic augite. 

 Viewed under a high power, the ground-mass is seen to be crowded with delicately- 

 shaped prisms of augite, most of which are terminated at both ends by faces of the 

 hemi-pyramid. 



In addition to the minerals already enumerated, apatite is present in fine needles, 

 and there are a few patches of a bright green chlorite, with which brown mica is 

 occasionally associated. 



The inter-relations of the various constituents indicate that they were probably 

 formed in the following order: — iron-ore and apatite, olivine, augite, plagioclase, and 

 finally, the unstriped felspar occurring in patches in the ground-mass. 



The rock was analysed some years ago for the Geological Survey by Mr J. S. Grant- 

 Wilson : — 



Si0 2 . 













4601 



A1 2 3 - 













19-19 



Fe 2 3 - 













591 



FeO . 













6-75 



MnO . 













19 



CaO . 













8-68 



MgO . 













6-81 



K 2 . 













1-20 



Na 2 . 













3-27 



H 2 . 













3-07 





101-08 







Sp. gr 



avity = S 



•8. 







The Olivine-Basalt of Hailes Castle* — This rock has a more felspathic character. 

 It consists chiefly of felspar microlites and slender laths, with granules of augite and 

 particles of magnetite. In this ground-mass lie isolated limonitic pseudomorphs after 

 olivine, and a few glomero-porphyritic crystals of striped felspar. A basalt occurring at 



* No. 630 of the Survey Collection. 



