28 REPORT OF NATIONAL MUSEUM, 1889. 



second head are included such forms as the sandstone concretions shown 

 from Arkansas (37600) and New Jersey (38761 and 38427). These are 

 due not to original deposition in concentric layers, but to the oxidizing 

 and leaching action of meteoric waters acting on nodular inclusions of 

 iron sulphide, or possibly carbouates. The oxide thus formed segre- 

 gates in zones, and by its cementing action binds the sand grains to- 

 gether leaving the central portion, formerly occupied by the pyrite, either 

 empty or partially filled with loose sand ; such forms are shown intact 

 and broken in halves in specimens 38427 and 37004. 



A zonal banding or shelly structure closely simulating concretionary 

 structure is common in rocks more or less weathered and decomposed, 

 but which is due not to original deposition or crystallization of mineral 

 matter about a center, but rather to the weathering of jointed blocks, 

 the various chemical and physical forces acting from without inward. 

 This is here illustrated in specimen 38570 from Montana (see also under 

 head of Rock- weathering). 



Still another form of concretion due to segregating forces acting 

 together with pressure are shown in the so-called cone-incones (38838, 

 39122, and 39175). 



(2) Joints. — Jointing on a sufficiently large scale to be appreciable as 

 a structural feature of the earth's crust can well be shone only by pho- 

 tographs and models. Here are exhibited a few isolated examples of 

 jointing in both sedimentary and igneous rocks. Special attention may be 

 called to the large basaltic column from the Yellowstone National Park. 

 The exhibit is at present meager, and is supplemented by a few photo- 

 graphs. Jointing in granite and illustrating its utility in quarrying is 

 shown in a photograph of the Red Beach granite quarries near Calais, 

 Maine (73457). Columnar jointing in volcanic rocks is shown in a pho- 

 tograph of " Rooster Rocks, " on the south shore of the Columbia River 

 near Portland, Oregon (73458); in a view near Mount Davidson, Cali- 

 fornia (38283) ; and two views of the Regla Cascades in Mexico (38301 

 and 38305). 



(3) Inclination, strike, and clip are subjects as yet scarcely touched 

 upon. From necessity these phenomena can be illustrated satisfac- 

 torily only by means of models and photographs. Emmons's sectional 

 model of Leadville and vicinity may be referred to here. 



(4) Curvature, cleavage, and distortion are quite well illustrated in 

 the series of distorted gneisses and schists shown in the cut and pol- 

 ished slabs from Auburn, Maine (39059) ; Brandon, Vermont (39018) ; 

 New York (39124); the roofing slates from Pennsylvania (70104-70107), 

 and the crushed pebbles already referred to under the head of " Effects 

 of pressure." The sharply foliated schists from Dutch Island in Narra- 

 gansett Bay are also worthy of mention (38608) as well as the large slab 

 of curved slate showing junction with shale (specimens 70102 and 70103 

 from Pennsylvania). 



(5) Igneous rocks as structural features in the earth's crust must on 



