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VI. — The Igneous Geology of the Bathgate and Linlithgow Hills. Part. II. 

 Petrography. By J. D. Falconer, M.A., B.Sc, F.G.S. (With Three Plates.) 



(Read March 5, 1906. Issued separately August 13, 1906.) 



CONTENTS. 



PAGE 



1. The Carboniferous Lavas . . .133 



Introduction ...... 133 



The Mineralogical Characters of the Lavas 134 



The Porphyritic Constituents . . 134 

 The Groundmass . . . . .134 



Structure, Classification, and Chemical Com- 

 position 135 



2. The Contemporaneous Intrusions . . . 137 



3. The Later Intrusions 137 



Introduction 137 



PAGE 



The Mineralogical Characters of the Diabases 138 



The Intersertal Material or Mesostaais . 144 



The Structure of the Diabases . . . 145 

 The Diabase Aphanites and Diabase Porphy- 



rites ....... 145 



The Segregation Veins .... 147 



The Chemical Composition of the Diabases . 147 

 Conclusion . . . . . . .148 



4. Acknowledgments 149 



5. Description of Plates 



150 



1. The Carboniferous Lavas. 

 Introduction. 



The lavas of the Bathgate and Linlithgow Hills occur, as already described,* in a 

 series of zones alternating with sedimentary deposits. So far as their field characters 

 are concerned they may be grouped with convenience into two classes : fine-grained, 

 columnar, basaltic types, usually porphyritic with augite and olivine, rarely with felspar, 

 and coarser-grained, doleritic types, usually much decomposed, not evidently porphyritic 

 or porphyritic with olivine alone. The yellow crusts of the compact lavas are minutely 

 vesicular and pumiceous, while steam-cavities are rare in the interior. The doleritic 

 lavas on the other hand are coarsely vesicular and amygdaloidal above and below, and 

 frequently also throughout. The blue basaltic types are relatively very fresh ; the 

 doleritic types are frequently entirely decomposed into a whitish, earthy material, with 

 knots of limonite, calcite, and quartz, similar in many respects to the white trap of the 

 coal-fields. Good examples of this mode of weathering may be found in the Eiccarton 

 Burn. The differences in texture are probably to be referred not so much to differences 

 in chemical composition as to the effect of variation in the quantity of water vapour 

 contained in the successive flows. The coarse and open structure of the dolerites has 

 evidently also given freer scope to the action of decomposing influences than the 

 more compact structure of the basalts. Both types are much veined by such secondary 

 minerals as calcite, siderite, limonite, quartz, chalcedony, and various zeolites. Fre- 

 quently cavities in the veins, steam-holes in the pumiceous crusts, and even vesicles 

 within the solid rocks, are found filled with brown viscous pitch or black lustrous asphalt. 



* Falconer, Trans. Roy. Soc. Edin., vol. xli., 1905, p. 359. 

 TRANS. ROY. SOC. EDIN., VOL. XLV PART I. (NO. 6). 



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