8 St. J ago. 



PAET I, 



The second metamorphosed variety is likewise a hard 

 rock, but without any crystalline structure. It consists 

 of a white, opaque, compact, calcareous stone, thickly 

 mottled with rounded, though regular, spots of a soft, 

 earthy, ochraceous substance. This earthy matter is of 

 a pale yellowish-brown colour, and appears to be a mix- 

 ture of carbonate of lime with iron ; it effervesces with 

 acids, is infusible, but blackens under the blowpipe, and 

 becomes magnetic. The rounded form of the minute 

 patches of earthy substance, and the steps in the progress 

 of their perfect formation; which can be followed in a 

 suit of specimens, clearly show that they are due either 

 to some power of aggregation in the earthy particles 

 amongst themselves, or more probably to a strong at- 

 traction between the atoms of the carbonate of lime, 

 and consequently to the segregation of the earthy ex- 

 traneous matter. I was much interested by this fact, 

 because I have often seen quartz rocks (for instance, in 

 the Falkland Islands, and in the lower Silurian strata of 

 the Stiper-stones in Shropshire), mottled in a precisely 

 analogous manner, with little spots of a white, earthy 

 substance (earthy feldspar ?) ; and these rocks, there 

 was good reason to suppose, had undergone the action of 

 heat, — a view which thus receives confirmation. This 

 spotted structure may possibly afford some indication in 

 distinguishing those formations of quartz, which owe 

 their present structure to igneous action, from those pro- 

 duced by the agency of water alone : a source of doubt, 

 which I should think from my own experience, that 

 most geologists, when examining arenaceo-quartzose dis- 

 tricts, must have experienced. 



The lowest and most scoriaceous part of the lava, in 

 rolling over the sedimentary deposit at the bottom of 

 the sea. has caught up large quantities of calcareous 

 matter, which now forms a snow-white, hiorhlv crvstalline, 



