14 REPOKT OF NATIONAL MUSEUM, 1889. 



inented by a series of photographs of the extinct volcanoes about Mono 

 Lake, California, as taken by Mr. I. C. Eussell (70320-70328). 



The non-volcanic igneous injections lend themselves less readily to 

 exhibition purposes owing to the size of the materials. Among the 

 more interesting objects now displayed are the basic trapdikes cutting 

 coarse granite from Auburn and Norway, Maine (35964 and 35520), and 

 the small kersahtite dikes cutting crystalline limestone from Franklin 

 Furnace, New Jersey (39041). The character of the material thus 

 formed may be best comprehended by reference to the specimens in 

 the rock collection proper. 



The subjects of earthquakes and secular upheaval and denression can 

 bo treated only by means of models aud illustrations. The scheme has 

 not as yet been worked out in detail. 



HYPOGENK CAUSES OF CHANGES IN THE TEXTURE, STRUCTURE, AND COMPOSITION 



OF ROCKS. 



(1) The effects of heat. — Among the more interesting and instructive 

 objects here exhibited attention may be called to the series illustrating 

 what is known as contact metamorphism, i. e., rocks which have been 

 changed or metamorphosed from their original condition by the heat 

 and the incident chemical action of intruded igneous rocks. This series 

 at present includes limestone in contact with a dike rock (39042) and 

 rendered white and coarsely crystalline thereby; clay changed to por- 

 celainite or porcelain jasper (3809) ; chalk rendered hard and almost vitre- 

 ous by the same agencies (39121); an argillite indurated and changed 

 to a bright red through the heat of burning beds of lignite (70675) ; 

 and a considerable series of sedimentary rock from various European 

 localities (principally the Vosges and Harz Mountains) which have been 

 altered to the condition known as fruchtschiefers, knotenshiefers, horn- 

 stones, spilosites, etc. (See Geikie, pp. 564 and 565.) The dike of ker. 

 santite in contact with zinc ore from Franklin Furnace, New Jersey 

 (39105), is peculiarly instructive, the heat having been sufficient to fuse 

 the ore at point of contact. In specimens 35932 and 35933 are shown 

 argillites from Nahant, Massachusetts, which have developed in them 

 greenish oval spots due to incipient crystallization caused by the pro- 

 trusion of a large mass of trap rock (diabase) shown in specimen 35931. 

 Specimen 36767, from near Deckertown, New Jersey, is a shale indu- 

 rated and banded by the intrusion of a large mass of uepheline syenite 

 in the vicinity. 



(2) The effects of compression, tension, and fracture. — A record of the 

 movements which have taken place in the earth's crust is kept by means 

 of the folds and faults developed. It is obviously impossible to show 

 these forces in actual operation, but something of their character and 

 force may be learned from an examination of the few specimens here 

 exhibited. The first to be noticed is a wax model made and presented 

 by Mr. Bailey Willis, of the XL S. Geological Survey (73180). Layers* 



