396 PROF. J. W. JIJDB ON THE TEETIAKT AND 



time which have intervened between the eruption of the older and 

 younger rocks respectively. 



In the Tertiary peridotites the several minerals, olivine, enstatite, 

 augite, picotite, &c., are perfectly fresh and unweathered ; but in 

 the Palaeozoic rocks these minerals are represented, in most cases, 

 only by their pseudomorphs ; and it requires the most careful study 

 to determine what was the nature of the original rock. When this 

 is done, however, we are impressed by the conviction that in minera- 

 logical constitution, as Avell as in structure, these Palaeozoic perido- 

 tites x^resent us with examples of all the varieties found among the 

 Tertiary peridotites. 



It may be convenient to apply distinct names to some of these 

 much altered igneous rocks, just as it is admissible to term the 

 indurated argillaceous sediments shales, while we call the less altered 

 rocks clays. But it is a fact which cannot be too strongly insisted 

 upon that when due allowance is made for the effects of alteration, 

 operating during the enormous intervals of time which have separated 

 the eruption of the Palaeozoic and Tertiary peridotites, the agreement 

 in all the original and essential characters between the rocks be- 

 longing to these widely separated periods is of the most complete 

 character. 



§ 1. AlTEEATION OE the ITl^TIRALS IN THE PaL.+JOZOIC PeRIDOTITES. 



The most striking fact concerning the Palaeozoic peridotites is 

 that, as a rule, the whole of the original minerals of the rock have 

 been converted into their pseudomorphs. The bulk-analysis of the 

 rocks shows that they differ in composition from the Tertiary peri- 

 dotites by the addition of water, and the diminution, to some extent, 

 of the silica and certain of the bases. The olivines have been con- 

 verted into serpentine ; the enstatites are often represented by the 

 same mineral or by steatite ; the augites have become hornblendes, 

 and the felspars have similarly been changed to zoisite and other 

 minerals. 



These changes are of a totally different kind from those which 

 we have ■ seen to affect the minerals in the more deeply seated 

 eruptive rock-masses of the Tertiary period. Whether previously 

 in their t^^ical form, or in a more or less Schillerized condition, 

 these minerals of the Palaeozoic peridotites are equally affected by 

 changes of a totally different character and origin. In some cases 

 the change consists in the addition of water, and the conversion of 

 an anhydrous silicate into a hydrous one. In other cases, the 

 change appears to be a purely molecular one, the conversion of an 

 unstable mineral into a stable one. 



That these changes are produced at moderate distances from the 

 surface where the minerals are affected by the percolation of atmo- 

 spheric waters there cannot be any doubt. By the study of a suffi- 

 ciently large series of specimens it can be shown that the changes 

 in question have reached their maximum in those cases where the 

 exposure of the rocks to atmospheric influences has been greatest, 

 while more deeply seated portions of the rock remain comparatively 



