HOW AND WHEN ALTERATION TAKES PLACE. 15 



the latest geological periods; that is, a rhyolite, a trachyte, an andesite, or 

 a basalt of the Azoic or Palseozoic times, was the same when erupted as is 

 the rhyolite, trachyte, andesite or basalt of the present day, or of the Ter- 

 tiary age. The difference at present existing between these ancient and 

 modern forms — as the writer believes — is due to the greater alteration 

 which the former have suffered ; although possibly, a difference in the depth, 

 or some peculiar condition prevailing at the time of tlie consolidation of the 

 rock, may have had some influence in causing these differences. 



Under imiformly like conditions, alteration is proportional to the age, in 

 rocks of the same constitution and structure ; but when rocks of like char- 

 acter are subjected to the same agencies, for the same length of time, like 

 results would be produced, let the age be what it will. The original crust 

 and the eruptive rocks must, then, have furnished the material for all the 

 other rocks, directly or indirectly, except such as was derived fi-om water 

 and the atmosphere. To ti-ace these changes, and to follow the rocks in all 

 their variations is the work of the petrographer. As did Cuvier with fossil 

 bones, so may the lithologist reconstruct the original rock from the fossil 

 fragments of it found in other rocks. The presence of fragments of one rock 

 in another, however, is not to be taken as proof of difference of geological 

 age between them, unless it can be proved that the inclosed rock is of sedi- 

 mentary origin. A lava flow on a sea-shore would have its fragments in- 

 cluded in any rock then forming, and this would hold true of all volcanic 

 ejectments. A dike, also, passing through a rock forming on the shore, 

 would have all materials broken from it inclosed in the rock then forming, 

 but both would be of the same aa-e geologicallv, altIjouu:h differing in order 

 of time. 



In studying the alterations in rocks we ought not to confound the great 

 molecular cliano-es that <jo on throu<>;h the rock mass as a whole, and those 

 changes which are due to superficial weathering. The latter reproduce to 

 some extent the characters of the former, but go to gi'eater extremes, caus- 

 ing in the end destruction and disintesrration of the rock mass itself. Tlie 

 internal changes are apparently chemical or molecular changes in the whole 

 rock mass, instead of simple pseudomorphic changes of single minerals. In 

 no sense is metamorphism to be looked upon as extended pseudomorphism ; 

 for pseudomorphic forms are but an accident in the process of alteration, and 

 they may or may not occur, according to the amount of that alteration. All 

 the changes in rocks are to be explained by taking into consideration the 



