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DK. A. HOLMES ON THE TEKTIARY [vol. lxxh, 



of amygdaloiclal basalts remarkably similar to those of Mozambique, 

 while the upper included extensive flows of trachyte and phonolite, 1 

 Thus in Abyssinia the evidence that the amygdaloids preceded a 

 series of alkali-lavas is complete. Unfortunately, in Mozambique 

 the observations hitherto made do not admit of so definite a 

 deduction. 



Of the remaining alkali-lavas described in this paper, none were 

 found in situ. 



South of the Monapo, the occurrence of a narrow dyke of 

 andesite cutting a flow of amygdaloid leaves no doubt as to the 

 succession. 



A complete statement of the sequence of igneous rocks through- 

 out the district cannot be given, but in the following synopsis the 

 associations in three representative districts are enumerated. Names 

 marked with an asterisk are of lavas not yet known in situ : — 



Sanliuti District. Solcoto District. Monapo District. 



fSolvsbergite.* iEgirine-trachyte.* Tephritic pumice.* 

 J Picrite-basalt. 

 ■| Phonolite. 

 |_ Basalt. 



f Pyroxene-andesite.* 



H ornblende - ancle site . 



B. -\ Amygdaloidal and Amygdaloiclal and Amygdaloidal and 



vesicular basalts. vesicular basalts. vesicular basalts. 



Basalt sills and dykes. Basalt sills and dykes. 



The amygdaloidal and vesicular basalts (which extend through- 

 out the area) vary but little in respect of their basaltic frame- 

 work. Such differences as have been noted depend on the pro- 

 portion of brown glass, the sizes of crystals, and the degree of 

 vesicularity. The amygdales are occupied by agate, jasper, and 

 quartz, and by a variety of zeolites ; moreover, the composition 

 and the relative abundance of the aniygdale-minerals constitute a 

 second line of variation. 



With the exception of the dyke of andesite already mentioned, 

 all the dykes and sills encountered in the volcanic area are basaltic, 

 and can be closely matched by rocks occurring as surface-flows. 

 The reason for this is clear : the dykes and sills in most cases are 

 exceedingly narrow, and they appear not to have been long 

 separated by erosion from the overlying lava-sheets to which they 

 served as feeders. The larger Mocheha dykes are free from 

 vesicles; but all the other specimens collected contain a residuum 

 of glass, and are minutely vesicular and amygdaloidal. A note- 

 worthy feature is the general absence of zeolites from these small 

 intrusions. Taking the basalts as a whole, a continuous series can 

 be arranged, starting -with the compact Mochelia basalt, passing 

 through various vesicular and amygdaloidal types, and finishing 

 with glassy scoriaceous basalts. 



The lavas fall naturally into two main groups, designated as 

 A & B in the list of types given below (p. 233). For petrographic 

 description the following order will be adopted. In the cases 



1 ' Observations on the Geology & Zoology of Abyssinia' 1870, p. 182. 



