Vol. 50.] STRUCTURE OF CARBONIFEROUS DOLERITES AND TUFFS. 641 



The thin sections containing lapilli may be divided into two 

 classes, olivine-bearing and olivine-free. (The olivine is seldom 

 fresh.) Augite occurs in very few cases of the former (mostly in 

 outcrop 53) and in none of the latter. 



The lapilli of the first class are composed of a glassy base with 

 the addition of either : — 



(a) Olivine, augite, and plagioclase ; or 



(b) Olivine and plagioclase ; or 



(c) Olivine, plagioclase, and crystallites ; or 



(d) Olivine. 



The second class are composed of: — 



(e) Plagioclase in a glassy base ; or 



(/) Plagioclase and crystallites in a glassy base ; or 

 (g) Glass only. 



They sometimes contain magnetite in addition to the above mineral. 



In some outcrops we have only a, b, c, d, or g, as {a d g), etc., 

 whilst in others we have such combinations as (a d g), (6 / g), or 

 (defg). Felspar is the prevailing mineral in them, and augite 

 occurs the least frequently. These results differ from those obtained 

 by Penck 1 for the crystalline secretions in basalt-tuff. He classes 

 them into (1) Plagioclase and olivine ; (2) Augite, olivine, and 

 magnetite ; and (3) Olivine and magnetite. 



The cement is crystalline calcite, or a paste consisting of smaller 

 lapilli (and probably their decomposition-products), or a limestone 

 paste. There is a very small admixture of non-volcanic material. 

 This is almost entirely represented by the more or less rounded 

 lumps or pebbles of limestone, which sometimes contain fossils. In 

 outcrops 46 and 59 blocks of olivine-dolerite are found in the tuff. 

 In the remaining outcrops, except in 16, 18, 39, and 54, 1 have found 

 blocks with a more or less glassy base belonging to the olivine-bear- 

 ing class, and, like the lapilli of that class, they may be divided into 

 a, b, c, and d. In outcrop 56 the blocks have a roughly spheroidal 

 shape. 



The specific gravity of the blocks is greater than that of the lapilli- 

 tuff in which they are embedded, and if we compare blocks and 

 lapilli-tuff of similar microscopical structure from different outcrops, 

 we shall notice that the specific gravity of the blocks is always greater 

 than that of the tuff, and that the blocks in their turn have a lower 

 specific gravity than the dolerites of the district. 



It is interesting to compare the Derbyshire tuffs with those of 

 Carboniferous age in the Firth of Forth Basin, described by Sir 

 Archibald Geikie. 2 In the Scottish tuffs no microlites were found 

 such as those which are present in some modern volcanic tuffs. In 

 Derbyshire crystals of augite and felspar are found in the crystalline 

 cement of some of the tuffs. In the Scottish rocks the lapilli consist 



1 'Ueber Palagonit- und Basait-tuffe,' Zeitschr. deutsck. geol. Gesellsch. 

 vol. xxxi. (1879) p. 571. 



2 Trans. Roy. Soc. Edin. vol. xxix. (1880) pp. 513-516. 



