TIME OF METAMORPHISM. 61 



their assaults in these two periods, which must have been separated by 

 some millions of years. 



The field open to examination is much more limited in the case of the 

 Cambrian conglomerates than in that of the Carboniferous pudding- stones. 

 Moreover, the pebbles contained in the beds have been subjected to more 

 metamorphism. Nevertheless, the studies which have been made show that 

 the rocks of the fundamental complex had attained about their present con- 

 stitution before the beginning of the Olenellns epoch. These pebbles rep- 

 resent the granites, gneisses, quartzites, etc., of the rocks which are found 

 beneath the Cambrian beds, and show that the crystalline condition of these 

 deposits was approximately the same as it is now. It is evident, however, 

 that there were quartzites and other semimetamorphosed beds which afforded 

 waste to erosion in the Olenellus epoch, beds which are not recognizable as 

 in place in the district, and which perhaps, like those noted in connection 

 with the Carboniferous, have disappeared from the district. It is neverthe- 

 less clear that the greater part of the crystalline rocks of this district were 

 already not only in their present mineralogical condition, acquired during a 

 period when they had been deeply buried beneath other rocks, but had been 

 stripped by the erosive forces of this ancient covering. 



It must not be supposed that the whole or even the greater part of 

 the metamorphism which has taken place in this region had been accom- 

 plished by Cambrian or even Carboniferous time. While in certain districts 

 and with certain rocks this work seems to have been then completed, or 

 nearly so, in other parts of the field the action continued at least until after 

 the deposition of the Coal Measures strata. Thus the beds of the last- 

 named series in the region about Worcester and in that about Wickford 

 have been transformed to gneisses which, but for collateral evidence, could 

 not be recognized as having been, in their original state, the associates of 

 ordinary coals. It is evident that in this last-named field the process of 

 metamorphism has gone on with exceeding irregularity, certain parts of the 

 most ancient deposits — as, for instance, the fossiliferous strata of the Olenellus 

 horizon — being not much altered beyond the induration common to all 

 ancient flaggy layers, while but a few miles away conglomerates have been 

 so far converted into crystalline rocks that the original character of the beds 

 has been almost completely lost. 



