1'20 OVERLYING AND TRAP ROCKS. 



is inconsiderable. The peninsula of India, and South 

 America, present well-known examples of enormous 

 depth as of extent. 



Of their original forms and dimensions, we have 

 more reason to be doubtful than even of those of the 

 stratified rocks. Independently of those usual causes 

 of waste to which all rocks are subject., many of 

 them appear peculiarly susceptible of decomposition. 

 Hence their frequently insulated state, and the conse- 

 quent difficulty of conjecturing the nature of many of 

 the independent masses which occur. From this, and 

 from their peculiar relation to the stratified rocks y 

 arises the difficulty of determining their relative 

 periods of deposition. Excepting a few particular 

 cases, if it were asserted that the whole were the 

 work of one interval of time, it would not always be 

 easy to find arguments to refute the opinion. It is 

 repeating a former statement to say, that extensive 

 losses of the secondary strata are to be traced, and 

 that these never covered the primary in every part ; 

 whence a mass of the overlying rocks deposited on 

 such a surface, might be in contact, in one part, with 

 granite or gneiss, and in another, with one of the 

 latest of the secondary strata ; causing a fallacious 

 apparent distinction in the time of deposition. And 

 as these rocks present signs of extensive waste, 

 perhaps even in a greater degree than the stratified 

 ones, it is evident that two masses might be esteemed 

 distinct, and distant in origin, though originally one, 

 should the discontinuity occur between the portions 

 lying on the secondary and those lying on the primary 

 strata. The fact itself is common, and easily proved ; 

 particularly in the case of the insulated hill summits 

 just mentioned. To what extent indeed that waste may 

 have existed, it is impossible now to know ; so that it is 



