ORIGIN OP CRYSTALLINE ROCKS, 37 



§ "73. Without here entering into the details of their geognosy or their lithology, it is 

 sufficient to recall the fact that such basic eruptive rocks abounding in zeolitic minerals are 

 found, with many characters in common, from the time of the Cambrian or pre-Cambrian 

 Keweenian series of Lake Superior to that of the trias of eastern North America, the tertiary 

 of Colorado and the British islands, and the recent lavas of Iceland. The secreted minerals 

 of these rocks often occur in closed cavities in tufaceous beds, constituting amygdaloids, 

 and, at other times, in veins or fissures of considerable size. They are not, however, con- 

 fined to the tufaceous or recomposed detrital exoplutonic rocks, (which are sometimes 

 themselves hydrated and transformed into palagonite, as described by Bunsen in Iceland,) 

 but occur in veins and cavities in massive rocks, as is well seen in the diabase of Bergen 

 Hill, New Jersey, and the massive basalt of Table Mountain, Colorado, both remarkable 

 for their zeolitic minerals. 



^ 74. The accumulations of secreted minerals in these conditions are often consi- 

 derable in amount. Among other examples, it may be noticed that the zeolitic masses in 

 the amygdaloids of the Faroe Islands are sometimes three or four feet in diameter, and 

 constitute a large portion of the rock. Veins of laumontite in Nova Scotia attain breadths 

 of a foot or more, while some veins on Lake Superior, which are made up to a great extent 

 of zeolitic and related species, are two and three feet or more in breadth, and often of 

 considerable extent. The history of the chemical composition of the zeolite-bearing rocks 

 of Lake Superior, and of the changes which have taken place in their degradation from 

 the original eruptive mass, have been studied in detail by Pumpelly, with the help of the 

 previous analyses of Macfarlane, but cannot here be discussed.'*' 



§ '75. We must here notice the modes of occurrence of the zeolites of Table Mountain, 

 Colorado, as described in 1882 by Messrs. Cross and Hildebrand."" The upper forty feet of 

 a great flow of basalt, one hundred feet or more in thickness, shows many cavities, large 

 and small, described as more or less flattened and drawn out. Some of these cavities are 

 empty, while others are more or less completely filled by various zeolites, which are also 

 found in fissures in the mass and, in the case of analcite, in a conglomerate made up of 

 pebbles of basic eruptive rocks, underlying the bed of basalt. The zeolitic deposit often 

 appears as " a reddish-yellow sandstone-like material, which occurs in many of the cavities. 

 In the larger ones it takes the form of a floor, the upper surface being horizontal, and 

 the deposit may be several inches in thickness. Small cavities have been completely filled 

 with it, and it is clear that the deposition has taken place from the bottom of each cavity, 

 upward. In parts of South Table Mountain, especially, the same material has filled fissures. 

 Usually the lower part of such masses is composed of a reddish-yellow mineral in irregular 

 grains, which form a compact aggregate, in which lie isolated spherules of a similarly-colored 

 radiated mineral. These spherules are seldom more than two millimetres in diameter, and 

 are very perfect spheres. They increase in number upwards, and finally form the greater part 

 of the deposit. In one cavity, six or eight feet in horizontal diameter and about two feet 

 high, the deposit is quite different. Here the main mass is loosely granular, and is formed 

 chiefly by a bright greenish-yellow mineral, while a stratified appearance is produced by 



"» T. Macfarlane, Geological Survey of Canada, 1866, pp. 149-164 ; Pumpelly, Geology of Michigan, 1872. 

 part 2; also the same, on The Metaaomatic Development of the Copper-bearing Rocks of Lake Superior, Proc, 

 Amer. Acad., Boston, (1876) vol. xiii, pp. 253-309. 



^ Cross and Hildebrand, American Journal of Science, xxiii., 4.52, and xxiv., 129. 



