part 1] SPILITES IjST DERBYSHIRE. 15 



the Carlsbad law. Extinctions are straight, or with a very low 

 angle. 



The Tides well-Dale flow [9] is well seen in a small disused quarry 

 directly south of the North Fault, as mapped by Dr. Bemrose,i 

 where the specimens mentioned on p. 11 were collected. Both 

 vesicular and non -vesicular rocks are well exposed. The former 

 has already been briefly described : it consists mainly of a mass 

 of felspar-laths and microlites with flow-structure. They appear 

 to be chiefly oligoclase, but subordinate andesine also occurs. 

 Felspar-phenocrysts are absent. 



In the non-vesicular rock, which overlies the vesicular portion, 

 large felspar-phenocrysts are very abundant. The transition from 

 non-porphyritic to porphyritic rock is absolutel}^ abrupt, and a 

 clear line of demarcation, marked by the greater alteration of the 

 underlying vesicular rock, is well seen. There can hardly be a 

 doubt that the porphyritic rock is a distinct flow from the under- 

 lying spilitic bed. Its phenocrysts are frequenth^ very long and 

 lath-shaped. Untwinned inclividuals occur among them, and these 

 have straight extinction ; but most of the crystals have extinction - 

 angles which show that a felspar predominates which is more basic 

 than that in the vesicular portion of the exposure. The dominant 

 species appears to be andesine, but subordinate oligoclase also 

 occurs, and probably some orthoclase as well. Augite is present in 

 fairly fresh condition, moulded on the felspars, and it is clear that 

 this bed does not come within the range of the spilitic type of 

 these lavas. Nevertheless, here too, replacement of the original 

 felspar by alkali-felspar appears to have commenced along cracks 

 and fissures in the phenocrysts. 



The lava of Crich Cliff [15] can now be collected only on the 

 waste-heaps of abandoned mines. Examples of both types of 

 rock may be found. The comparative freshness of the 

 basalts is very striking, when compared with specimens having 

 spilitic affinities. In the former granular augite is abundant, and 

 remains of unaltered olivine may be seen. A few felspar-phenocrysts 

 occur, but the bulk of the felspar-content is in the form of laths. 

 Extinction-angles indicate labradorite as the dominant species ; but 

 untwinned water-clear crystals, with straight extinction and appa- 

 rently of secondary origin, occur in subordinate quantity. In 

 specimens with spilitic affinities from the same waste-heaps the 

 dominant original felspar is sometimes oligoclase, sometimes ande- 

 sine labradorite ; but replacement on a considerable scale has taken 

 place, and the other constituents are always destroyed. Here, too, 

 untwinned laths and Carlsbad twins with straight extinction occur. 



The lava of Worm Wood, near Bake well, is a good exam])le of 

 a rock intermediate between those of spilitic type and the true 



1 Q. J. G. S. vol. Iv (1899) map, pi. xix. 



