172 



All over Australia and ia the East Indies scattered over the 

 surface, or entombed in alluvial drifts, are found curious little 

 button-shaped bombs of black volcanic glass. The origin of 

 ■ these is not understood, but it is now generally believed that they 

 have come from some source beyond the Earth — in other words^ 

 they are glassy meteorites. Specimens are shovpn from Uralla, 

 Braidwood, Cobar, Barrier Eange, Central Australia, Yictoria,. 

 and West Australia. Those from the alluvial leads at Uralla are 

 beautifully preserved ; they seem to have fallen into the water, 

 and were then covered up by alluvial material which has protected 

 them from damage.* 



Alteration Effects. — The effect of a mass of molten rock coming 

 in contact with sedimentary strata is to harden the latter, and 

 sometimes to produce a columnar structure. In this way coal 

 may be coked, sandstone may be converted into quartzite, and 

 shale into lydian stone or jasper, and all three may be rendered 

 prismatic. Similarly limestone becomes marble. Coked Coal is 

 shown from BuUi, Mittagong, and India. The Mittagong 

 specimen is also prismatic. At the end of case 63 is a most 

 instructive specimen showing a tongue of igneous rock intruding 

 coal : the coal is prismatic all round the tongue (Plate Yll). 



Columnar baked shale from East Maitland in case 138. 



Columnar sandstone occurs near Sydney (see page 186). 



Columnar jointing. — Igneous rocks on cooling very frequently 

 assume a beautiful columnar structure. The columns may be 

 scores of feet long and several feet in diameter. This is well 

 shown in the photograph of the Kiama blue-metal quarries, on 

 th.e north w^all. 



A column of basalt, that has been used as a landmark, has 

 been mounted in case 131. Columnar porphyry from Cowra, 138. 



ETiscellaneous. Case 112. — ■T'oraminiftral Sands. 



Case 111 — Coal Nodule. — The origin of these rounded masses 

 of coal is not understood. In the English Coal Measures they 

 frequently contain remains of fossil plants most beautifully 

 preserved. 



* It is much to be wished that miners and others would interest themselves in these objects, 

 and make an exact note of the position in which they arejoimd. 



