124 THE C1;YSTAL falls IRON-BEAKING DISTRICT. 



typical occurrence in Hawaii is found only in dry places, whereas the aa is 

 confined to those ])arts of the lava stream — which in other portions of its 

 course is perhaps developed as pahoehoe — where it crosses moist valleys or 

 other depressions presumed to have contained a considerable amount of 

 moisture.^ 



In the case of some of the block lava of Santorin described by Fouqucy 

 with which this may be com23ared, the conditions were such that the lava 

 practically welled up through the water. 



From Dana's description it appears that lava in the pahoehoe form can 

 not exist in the presence of moisture, being changed to the aa form. It 

 would thus seem that Teall's statement of a submarine origin for the 

 pahoehoe lava is untenable. 



Wherever the ellipsoids have been studied in the Crystal Falls district, 

 they have been found to exist as separate units, thus indica,ting the extremely 

 viscous character of the lava. It would seem that the analogy between 

 these basalts and the aa or block lava is much greater than that which 

 exists between them and the pahoehoe or smooth-flowing lava. 



A.MYGDALOIDAL STKUOTUIiK. 



The amygdules in the basalts are composed of nearly the same min- 

 erals as those which occur secondarily in the rock mass itself. Arranged 

 in order of frequence of occurrence, they are as follow^s: Chlorite, epidote- 

 zoisite, quartz, calcite, feldspar, iron oxide, and biotite. An amygdule may 

 consist entirely of one of the above minerals, or, as is most commonly the 

 case, of two or more of them. In the latter case the minerals are usually 

 arranged in concentric layers. The nonoccurrence of zeolites is very 

 noticeable. Their absence from these Huronian A'olcanics is especially 

 striking since they are so common in their altered modern equivalents, 

 and also occur in basalts as old as those of the Keweenawan of Lake 

 Superior^ and of the South Mountain of Pennsylvania.^ 



' Cf. Characteristics of A'^olcanoes, by J. D. Daua : New York, 1890, p. 243. 



= 0p. fit., Chap. II. 



■'Paragenesis and derivation of copper and its associates on Lake Superior, by Raphael Pum- 

 pcUy : Am. .Jour. Sci., 3d ser., \o\. II, 1871, p. 188; also Geol. Survey, Michigan, A'ol. I, part 2, 1873, pp. 

 19-46; Geol. of Wisconsin, Vol. Ill, 11-80, p. 31. 



The copper-bearing rocks of Lake Suiierior, by E. D. Irving: Mon. U. S. Geol Survey, Vol. V. 

 1883, p. 89. 



■iTbe volcanic rock.s of South Jlonntaiii in Pennsylvauia ami Maryland, by G. H. Williams: Am 

 Jour. Sci., Sd ser.. Vol. XLIV, 1892, p. 4iU. 



