LUNOA. GEOLOGY. 1 6? 



intervention of some substance differing from the pre- 

 dominant one, or to arise from some change of structure 

 in the consecutive strata. 



The simplest substance among these beds is an extremely 

 compact and crystalline granular quartz, varying from 

 white or pale grey to a darker blueish grey, and scarcely 

 yielding to the impressions of the atmosphere ; although 

 the coloured varieties become bleached to perfect whiteness 

 at the surface and to a small depth within it ; in con- 

 sequence of some chemical change of the colouring 

 principle which was lately noticed in treating of the 

 Slate isles. The change to whiteness takes place here, 

 as in the trap veins of those islands, through the inter- 

 vention of a brown lamina; the protoxide, or carbonate 

 of iron, being converted into rust before the colour is 

 finally discharged. It is difficult to understand how 

 water can procure access beyond the immediate surface 

 of a substance so compact as this rock. 



The grains of quartz occasionally become larger, and 

 a coarse compound is thus formed ; while, in other cases, 

 coarse gravel and fine sand are irregularly mixed in the 

 same bed ; the whole being condensed into a semi-crys- 

 talline mass of extreme hardness. 



These simpler rocks, whether fine or coarse, alternate 

 in frequent changes with other beds, which consist of the 

 same substances as bases, intermixed with fragments, both 

 of micaceous schist and of clay slate that vary much 

 in size, and sometimes attain even the length of an 

 inch. In these instances, the compounds resemble the 

 well known sandstones which occur among the secondary 

 strata and contain fragments of schist ; and which differ 

 from these, chiefly in the absence of the crystalline texture, 

 and in the inferior hardness of the base. As these com- 

 pounds become finer, a series of rocks is gradually 

 produced, terminating in an aggregate of quartz grains 

 cemented by schist, and not to be distinguished from 



