1819.] Greenongh on the First Principles of Geology. 305 



the two cases. The same diversity of opinion exists with respect 

 to the granite rocks in our own country. Prof. Jameson 

 describes Goatfield as stratified, Prof. Play fair as unstratified. 

 Prof. Playfair says, that Mount Sorrel is stratified, while Lord 

 Webb Seymour affirms that it is unstratified. 



The same diversity of opinion exists with respect to the stra- 

 tification of sienite, porphyry, hornblende, greenstone, serpen- 

 tine, transition limestone, siliceous slate, and greywacke. 



Our author endeavours to account for this diversity of opinion 

 by the different ideas which different writers affix to the word, 

 or by appearances from which they do not deduce the same 

 conclusions. These are chiefly the following : 



1. Some rocks present external planes parallel to each other ; 

 but are not subdivided by internal planes. I presume that the 

 author means, that in some cases when we survey the face of a 

 rock, we observe lines parallel to each other at certain intervals, 

 indicating at first sight that the rock is divided into different 

 layers or beds ; but upon examining these lines, we find them 

 to be merely superficial, and not the terminations of planes 

 penetrating through the body of the rocks, and dividing it into 

 different beds. When this occurs, as is sometimes the case, it 

 is obvious that an observer may be easily so far misled as to 

 ascribe stratification to rocks which do not possess it. 



2. Parallel internal planes are not always coextensive with the 

 rock in which they occur ; that is to say, when we survey the 

 face of a rock, we sometimes observe parallel lines, which are 

 the terminations of planes dividing the rock into layers ; but 

 these planes do not penetrate through the whole rocks, but 

 terminate at unequal distances from the exposed surface ; so 

 that the rock appears partly stratified, partly unstratified. In 

 such a case, the stratification must be illusory. 



3. Parallel planes are sometimes too distant from each other, 

 and sometimes too near each other, to give the rock in which 

 they occur an undisputed claim to stratification. This obviously 

 depends upon the meaning affixed to the term stratijication. If 

 it signifies deposition from a liquid, the restriction is correct. 

 When it is said that Mont Blanc consists of three vertical beds 

 of granite, it must be obvious that these beds could never have 

 been mechanically deposited from the sea. In the same way, 

 though mica-slate and clay-slate are composed of flakes lying 

 above each other, yet this slaty structure is not of itself suffi- 

 cient to demonstrate that these rocks were mechanically depo- 

 sited from the sea. 



4. Two or more sets of parallel planes sometimes occur in the 

 ue rock, so as to meet perpendicularly or obliquely. This 



I'.ppearance, as far as I have observed it, occurs in limestone and 

 gypsum, :uid seems to depend upon crystallization. 



6. In rocks that have the appearance of deposition, the planes 

 of the strata are not always parallel. We should expect this ta 



Vol. XIV. K° IV. U 



