SoLLAs — On the Structure and Origin of Quartzite Rocks. 173 



Hence their stratigrapliical relations in no way suggest, but on 

 the contrary strongly oppose, the idea of their being brought into 

 their present position by the folding of any bed. They are for the 

 most part entirely composed of quartz ; only one or two, which 

 must be referred to the same group, contain a trace of other 

 minerals. By this character they are completely cut off from all 

 the other rocks of quartz, even the whitest quartzite in a bedded 



form, such as the Porth-y-gwalch, being much less pure 



From the banding of portions of the rock, from its purity, and 

 from the mode of its occurrence, it has been suggested that such 

 a knob represents the underground base of a hot spring of the 

 period." When the quartzite contains evidently rounded grains, 

 the presence of which Professor Blake admits in certain cases, he 

 asks : " Have these been worn by the very water from which the 

 matrix has derived its silica ? and has the attrition taken place 

 not in streams on the surface, but in circulating currents under- 

 ground ?" 



Again, in Professor Blake's account of the " Monian " system 

 of rocks (10) we find the same views expressed ; thus on page 475 

 the following occurs : — " The quartz knobs are the most remarkable 

 features of all the Anglesey rocks, and their origin is a matter of 

 great difficulty. In their characteristic form they stand up as 

 isolated hummocks on the surface of the country, and are often 

 nearly as high as they are long or broad. They may be low 

 mounds or may rise as high as a house, or form a good-sized hill. 

 Usually they are elliptical in outline, but may be narrow and long, 

 though it is doubtful whether we should refer the latter to the 

 same category or consider them as ordinary veins. In structure 

 they may possess scarcely any clastic elements, or they may con- 

 tain many very rounded pebbles of pure quartz, the whole or 

 the remainder being composed of clear quartz in dusty-looking 

 polygons of growth separated by clear lines, the dusty appearance 

 being due to excessively fine cavities, the largest of which may 

 possess fluid enclosures. The only impurity is a little occasional 

 sericite which may form round the larger pebbles where the matrix 

 is not quite close. The purity and structure of the rock, together 

 with its mode of occurrence, force me to the conclusion that it has 

 been formed on the spot where it is now found, though the pebbles 

 may have been brought there along with the formative material. 



