138 



an inch ; and, in length, they are equally 

 uncertain. 



They are sometimes distinguishable in the 

 fresh rock, by difference of aggregation, or by 

 differences in the proportions or sizes of the 

 parts, occasionally accompanied by variations of 

 colour. In other examples, their existence is 

 only discovered after the decomposition of the 

 including rock ; when they remain traversing 

 the mass of clay or other materials which have 

 resulted from this cause. Indications of their 

 existence are also given, even where a rock has 

 not been disintegrated or decomposed ; their 

 superior hardness causing them to protrude above 

 the general surface. 



They often, in this case, present a reticulated 

 appearance ; indicating that the interior of the 

 rock contains laminae crossing each other. 

 Examples of this structure are common, both 

 in granite, and in the sandstones. 



When such laminae are numerous, the inter- 

 vals form depressions or cavities ; and the venous 

 then passes into a cancellated structure which 

 may be included with the following. 



