ERUPTIVE ROCKS. 293 



Here, as in all attempts at establishing geological classiiication, there 

 are found to be occurrences which form intermediate or transition 

 members between tlie two groups, but yet do not invalidate the legitimacy 

 or advisability of establishing such a division or classification. Geologists 

 have hitherto been divided in opinion as to the nature of the relation be- 

 tween the age of an eruptive rock and its internal structure and compo- 

 sition, the extremists on one side maintaining that this relation is an abso- 

 lute and fixed one, and that where, as is so often the case, its geological 

 and external structural relations furnish no evidence as to the age of a rock, 

 a careful study of its internal structure is sufficient to determine within 

 certain limits its period of eruption; those of the opposite school maintain 

 that no such relation exists, that the correspondences observed are merely 

 accidental coincidences not dependent on age, and that the many sub- 

 divisions established by the former school are not legitimate, and, inasmuch 

 as there are an infinity of intermediate members, could with advantage be 

 reduced to a few general divisions. The nuxnner of eruption, whether as 

 an intrusion or as a surface flow, has not in general been considered an 

 essential function of classification. In this region it would seem that the 

 characteristic differences of internal structure of the above two classes de- 

 pend rather upon the conditions under which they have consolidated than 

 upon the absolute geological age of either class, altliough their relative ages, 

 which are distinctly marked, correspond, as it happens, with the cliffering 

 conditions of consolidation. 



Age. — As regards their period of eruption, the rocks of this region may 

 be divided into an older and a younger series, the former of which were 

 erupted before the dynamic movement which caused the uplift of the range 

 and were involved with the inclosing sedimentarj'' strata in the consequent 

 folding and faulting, while the latter are of later date than that dynamic 

 movement. 



An exact definition of the age of either group is unfortunately not yet 

 possible. In the first place, the time of the dynamic movement is assumed as 

 at the close of the Cretaceous period, this assumption having been adopted 

 by the consensus of tlie geologists who have studied the Rocky Mountain 

 region, for the reason that the Tertiary beds, where found in contact, are 



