GEOLOGY. 239 



pressure and tension, probably at a moderate elevation of tempera- 

 ture (but not necessarily j^o), had been sufficient to produce the 

 molecular movement ot these hard and appaicntly iinyicldiiig ma- 

 terials. Mechanical force alone appeareil to have been the a<i;(nt, 

 and M. Tresca had shown that under enormous pressure solids 

 could be made to flow in the same manner as liquids, or that in 

 their movements they lollowed the same law. The remainder of 

 the paper pointed out certain facts and illustrations tending to 

 strengthen these views. By the careful study of these phenomena 

 of plasticity new views were opened of the structure of great rock 

 masses; of the phenomena of plication, lamination, and of the 

 origin of some structural peculiarities of mineral veins and their 

 enclosing; walls. In view of all the facts, Prof. Blake thought that 

 geologists should admit that very great changes had been pro- 

 duced in the structure of rock masses by simple mechanical pres- 

 sure, unaided by any great elevation of temperature, or by ex- 

 traordinary chemical agencies. 



]\Jr. J. B. Perry exhibited some pebbles obtained at Purgatory, 

 in which one pebble appeared driven into another, showing the 

 eflects of this kind of compression. 



Mr. T. Sterry Hunt made a few remarks bearing on these points. 



HINTS ON THE STRATOGRAPHY OF THE PALAEOZOIC ROCKS OF 



VERMONT. 



Prof. J. B. Perry, at the meeting of the American Association, 

 read a long and elaborate paper on the^bove subject, throwing 

 much light on many geological phenomena connected therewith. 



Prof. Agassiz said that Prof. Perry iiad stuck to a difficult sub- 

 ject with unusual perseverance, and had, he thought, solved it to 

 the satisfaction of all those who should afterwards critical Iv g-o 

 over the ground. He said that Prof. Perry \s object was to cstai)- 

 lish the precise stratification of the rocks in question and their re- 

 lation to each other, in order to prevent future mistakes of palae- 

 ontologists in mixing up fossils of different periods. He thought 

 American geologists were under an obligation to him for what he 

 had done. 



FLORA AND FAUNA OF THE MIOCENE TERTIARY BEDS OF OREGON 



AND IDAHO. 



Prof. J. S. Newberry exhibited, at the meeting of the American 

 Association, a beautiful series of fossil plants collected by Rev. 

 Mr. Condon, of Dallas City, Oregon. 



These plants, the professor said, were from the fresh-water de- 

 posits which cover so large a surface of the Great Basin in Neva- 

 da, Idaho, and Oregon, and were of special interest both from 

 their geological position and botanical character. They were 

 contained in the sediments deposited by a series of great fresh- 

 water lakes which once existed in the area lying between the 

 Rocky Mountains and Sierra Nevada. 



