284 BULLETIN OF THE 



those geologists who hold that conglomerates have been metamorphosed 

 into felsites, have forgotten that conglomerates -are usually mixed rocks, 

 while in the cases cited by them the pebbles are felsites, the same as 

 the felsite into which they are said to be changed. We should expect, 

 when an outflow takes place, that there would be a re-heating and 

 breaking up of the old lava in the vent, which would naturally give us 

 this brecciated material. However old the rocks may be, the materials 

 of the first class in each species remain the same in character, from the 

 earliest ones that I have examined down to those of the present time. 

 The alterations to which volcanic rocks and their derived sediments 

 are subjected make it often difficult to distinguish one from the other ; 

 but this difficulty arises, not from the sedimentary rock assuming, under 

 any agency to which it is naturally subjected, the form of a volcanic 

 one, except so far as it retains the characters and altei-ations belonging 

 to its parent rock, but from the volcanic one taking fx-om alteration, 

 characters closely like those of sedimentary rocks. I have never found, 

 under the microscope, the sedimentary characters obliterated in the 

 most highly metamorphosed rocks that were known to have a sedi- 

 mentary origin ; and, furthermore, the minerals and mineral varieties 

 of the third class in sedimentary rocks are, with possibly one or two 

 exceptions, never like the minerals arising from the crystallization of 

 any lava. The supposed passage of a sedimentary rock into an eruptive 

 one can only be considered proved when it is shown positively that the 

 observer knows the nature of the rocks in question, and tliat he has 

 examined every inch of the portion lying between them, so as to be able 

 absolutely to prove that no line of junction can exist. One such case 

 worked in that way would prove the entire question in favor of the 

 popular geological school. 



The Geological Exploration of the Fortieth Parallel adopted, with mod- 

 ifications, Richthofen's classification of the Tertiary and recent volcanic 

 rocks, and Mr. King, in his report, states that each of these rocks shows 

 always a basic, mean, and acidic form. He classes rhyolite as the acidic 

 form of basalt, placing both under the head of neolite. As my exami- 

 nation of Mr. King's collections convinces me that his system is based 

 on errors, it is well to point out briefly some of these, taking, of course, 

 the report of Professor Zirkel as the basis, and following, in a measure, 

 its order. 



The mica schist that Professor Zirkel calls a paragonitc slate, like 

 that from St. Gotthard, is similar to many mica schists in New England, 

 and, except that its color is grayish-white, has no resemblance to the 



