FROM Till-: CROSS FELL INLIER. 525 



I'rom this neighbourhood (marked ' Spring ' on the six-inch map) 

 has in addition a quantity ot' finely crystalline to cryptocrystalline 

 silica, sometimes with a spherulitic structure [1320]. In the field 

 this rock presents a singular appearance owing to the numerous 

 parallel narrow veins of calcito which traverse its dark mass. 



A specimen of diabase from a dyke in Hake Beck difi'ers in some 

 respects from the Guns Fell rocks. The felspar seems to approach 

 typical labradorite, and encloses occasional crystals of light-brown 

 sphene [923]. 



At Deep Slack Wood, close to the exposure of the basic lavas of 

 Mclmerby, occurs a fresh-looking, finely crystalline dolerite, very 

 different from the neighbouring rocks. It consists largely of little 

 lath-shaped striated felspars, besides a few larger individuals with 

 broader lamellas and an occasional shapeless felspar with strong 

 zonary structure, of the kind so characteristic of doleritic rocks 

 [1323]. The augite forms ophitic plates each enclosing many of the 

 small felspars ; magnetite occurs interstitially among the felspars in 

 some abundance. The felspars are always quite clear; much of 

 the augite is fresh, but part is replaced b}^ brown and green decom- 

 position-products, and a similar green substance fills the few 

 scattered vesicles. The rock is unlike any known Ordovician lava 

 or intrusion in the district, and suggests a dyke of much later age, 

 post-Carboniferous or even perhaps Tertiary. 



7. QUARTZITE OF EoMAN FeLL. 



An interesting modification of the Carboniferous sandstone is 

 represented by some of Mr. Marr's specimeiis collected on Koman 

 Fell. The sandstone here has been locally converted into a compact 

 vitreous-looking rock by the deposition of secondary quartz. 



Under the microscope [911] the rock exhibits all the characters 

 of a quartzite such as those of Hartshill and the Stiperstones *. 

 The original grains are almost all well rolled. Quartz largely pre- 

 dominates, mostly with a very dusky appearance due to crowds of 

 inclusions, often ranged in parallel lines. A few of the quartz- 

 grains are composite. In much less quantity occur weathered 

 felspar-fragments and little rounded pieces of a microlithic andesite. 

 The interstitial quartz occurs entirely as a •' secondary enlarge- 

 ment," easily distinguished from the original grains, with which it 

 is in crystalline continuity, by its clear appearance. More rarely 

 the felspar grains show a similar secondary growth, a phenomenon 

 observed elsewhere by Yan Hise and others. It occurs both on 

 orthoclase and plagioclase-fragments. 



* Compare, e. g., the Lickey quartzite figured in Mr. Teall's 'Brit. Petrogr.' 

 plates xIt., xlvi. 



