THE SHAr GUANITE AND ASSOCIATED ROCKS. 321 



amount of carboiiates of lime and magnesia in the original rock. 

 The metamorphism is evidently incomplete, and the clastic grains ot 

 quartz show no change. 



The Coniston Grit and the Lower Coldwell beds (or grits in tlie 

 Coniston Flags) resemble one another very closely. They are 

 ordinary grauwacke grits. A specimen of the former, taken near 

 Stakeley Folds, shows in a section subangular grains of quartz and 

 felspar with some interstitial dusty matter like kaolin, and little 

 patches of finely granular calcite. The felspar has minute twin- 

 lamellation and is rather abundant, though subordinate to the 

 quartz. There is no other clastic element except very rarely a flake 

 of white mica. The rock is freely veined with quartz [1165]. 



For comparison we take a specimen of the Lower Coldwell grit at 

 Packhouse Hill, about 600 yards from the granite-contact. The 

 contrast is evident in hand-specimens, the metamorphosed rock 

 showing the vitreous appearance of a quartzite, in which the 

 granular structure is only faintly discerned. Under the microscope 

 [1223] we see a mosaic of quartz and felspar, the irregular grains 

 of which show the " sutural" junction characteristic of crystal- 

 lization in situ. It is not easy to judge of the proportion of felspar 

 present, since the grains are ail perfectly clear, and twinning is 

 rarely seen. The twinning is never compound, and the grains 

 showing it give rather lower polarization-colours than the average, 

 which seems to ipoint to orthoclase. Besides these minerals there 

 are numerous little rounded brightly-polarizing granules, colourless 

 or very faint yellow, and precisely similar to those so commonly 

 seen in the metamorphosed Coniston Limestones. We regard these 

 as a lime-augite. The granules are aggregated together, especially in 

 irregular vein-like streaks. The slide shows also some small irregu- 

 larly-shaped granular patches, so densely packed as to be opaque, 

 and appearing yellow in reflected light. These are, at least in part, 

 of the same pyroxenic mineral, which corresponds closely in its 

 distribution with the calcareous decomposition-product in the non- 

 metamorphosed grit. It is noteworthy that neither mica nor garnet 

 has been found. The kaolin seems to have gone with the carbonates 

 to form pyroxene. (See PI. XII. fig. 6.) 



The quartz in this rock encloses many irregularly-grouped minute 

 cavities, round or more frequently shapeless, with bubbles of various 

 relative size. Judging by the apparent relief of cavities and bubbles, 

 both glass- and fluid-pores may be represented, but no movement 

 was verified in any of the bubbles. 



The calcareous Middle Coldwell beds, as seen on the top of Pack- 

 house Hill, exhibit a high degree of metamorphism. As in the 

 Coniston Limestone, this is shown especially by the development of 

 lime-bearing silicates, and, although we have no analyses of these 

 Silurian strata, a comparison with the general character of the 

 unaltered beds makes it appear that a ver}' moderate proportion of 

 calcareous matter, which would not cause a field-geologist to 

 describe the rocks as limestones, is sufficient to make the meta- 

 morphism follow this line. 



