﻿part 2] 



PlCKlTE-TESCHEJSriTE SILL OE LUGAH. 



105 



I. Teschenite-basalt, upper contact of Lugar sill, Glenmuir Water, Lugav. 

 Chemical analysis by Dr. A. Scott. 

 II. Banded teschenite, upper tescbenite layer of Lugar sill, Bellow Water,. 

 Lugar. Chemical analysis by Dr. A. Scott. 



III. Banded teschenite, same as II, calculated from Rosiwal analysis No. 1 of 



Table I. 



IV. Teschenite, rich in analcite. upper teschenite layer, Bellow Water, Lugar.. 



Calculated from Rosiwal analysis No. 11 of Table I. 

 V. Teschenite, from lower teschenite layer, Glenmuir Water, Lugar. Calcu- 

 lated from Rosiwal analysis No. Ill of Table I. 

 VI. Teschenite, from lower teschenite layer, Glenmuir Water, Lugar. Calcu- 

 lated from Rosiwal analysis No. IV of Table I. 

 VII. Melauocratic teschenite, from upper teschenite layer, Bellow Water, Lugar.. 



Calculated from Rosiwal analysis No. V of Table I. 

 VIII. Teschenite, Mons Hill, Dalmeny (Midlothian). Analysis by E. G.Radley, 

 ' Geolosry of the Neighbourhood of Edinburgh ' Mem. Geol. Surv. Scotland,. 

 1910, p. 299. 



The teschenitic character of these analyses is at once evident. 

 For a silica percentage averaging 45, and high ferrous iron and 

 lime, the alkalies run to about 7 per cent. The magnesia is com- 

 paratively low, indicating the poverty of the rocks in olivine. 

 Combined water, due to the analcite present, is of course high. 

 The analyses obtained by calculation from the Rosiwal analyses 

 are remarkably in accord with the chemical analyses. The greatest 

 discrepancies are in magnesia, potash, and water. The latter may 

 be explained by the fact that no account of the alteration of the 

 rocks was taken in the calculation. The excess of magnesia may 

 perhaps be explained by the complete alteration of the olivine to 

 serpentine in the analysed rocks. The serpentine was analysed as. 

 serpentine, but calculated as olivine. The deficiency in potash is. 

 due, as explained above, to the difficulty of detecting and measuring 

 orthoclase in these rocks. The analysis of the Mons Hill teschenite 

 shows the essential identity of the Midlothian occurrences with 

 those of Western Scotland. 



The richness of some of the Scottish teschenites in potash points 

 to the fact that some of the analcite-rocks are probably to be 

 refen-ed to the monzonite series. With increasing abundance of 

 orthoclase teschenites pass into analcite-monzonites, analogous to 

 nepheline- and leucite-monzonite (sommaite). 



The abundance of lime in the teschenite-basalt (I) as compared 

 with the normal teschenite, is due partly to calcite, and partly to 

 a slight enrichment of the contact-rock in augite. The abundance 

 of olivine in the melanocratic type (VII) is sbown by a great 

 excess of ferrous iron and magnesia in the calculated analysis. 



(3) Theralite. (PI. X, figs. 3 & 4.) 



Three phases of the theralite may be distinguished. A black or 

 dark-grey, compact, doleritic rock forms the main mass of the 

 stratum. The lower part, however, carries abundant hornblende, 

 and is slightly coarser in grain ; while, near the junction with 

 teschenite, the rock is veined and shot with patches of a medium- 

 grained, light-grey, analcitic variety. 



Microscopically, the theralites consist of essential plagioclase,. 



