684 



jiefinite mineralogical and zoological types. Whether definite zoolo- 

 gical groups can be made out in any lower system still remains to 

 be seen. The rigid determination of the Devon and Cornish fossils, 

 which are very numerous, and a rigid comparison of theBerwyn and 

 Bala fossils with those near the base of the third group of the Cum- 

 brian section, give the fairest promise of an answer to the question, 

 and are pointed out as immediate desiderata. 



The difficulty of classification by organic remains increases as we 

 descend, and is at length insurmountable ; for in the lowest strati- 

 fied groups, independently of metamorphic structure, all traces of 

 fossils gradually vanish; and the great range of certain species 

 through numerous successive groups, and the very irregular distri- 

 bution of fossils even in some of the more fossiliferous divisions, add 

 greatly to the difficulties of establishing true definite groups even 

 within the limits of our island. The difficulties are indefinitely 

 increased in comparing the formations of remote continents. But 

 these circumstances are compensated by the magnificent scale of de- 

 velopment of the successive groups, and their wide geographical 

 distribution. Taken together they have a great unity of character ; 

 and even in remote continents they seem to form a common base, 

 from which we may hope to compute the whole series of secondary 

 and tertiary deposits that surmount them. 



The author then briefly touches on questions of structure and 

 cleavage ; on the indefinite alternations of trappean beds ; on meta- 

 morphic structure ; on the long duration of the deposits ; and on their 

 great disruptions and symmetrical dislocations, indicating a greater 

 violence of disturbing forces than is indicated in the secondary for- 

 mations of this country. Following the geological scale of deposits 

 from top to bottom, we can trace a series of phsenomena indicating 

 the same kind of causation differing at different times in intensity 

 and degree. The mean intensity can therefore only be collected 

 by ascertaining the intensity during every geological period, and can 

 never be obtained by assuming the intensity of any one epoch, past 

 or present, as the arbitrary standard. Again, the successive organic 

 types indicate great physical changes ; and following the descending 

 scale they at length vanish ; conducting us, however, to the confines 

 of other investigations in exact science which must prove the ulti- 

 mate basis of physical geology. 



Finally, the author gives a tabular arrangement of the several 

 classes and subdivisions agreeably to the system of the preceding 

 communication. 



Class I. — Primary stratified Groups. 

 Gneiss, mica slate, &c., &c. Highlands of Scotland and the 

 Hebrides. Crystalline slates of Anglesea and the S.W. coast 

 of Carnarvonshire. 

 The series generally without organic remains; but should organic 

 remains appear unequivocally in any parts of this class, they may be 

 described as the Protozoic system. 



