114 THE CRYSTAL FALLS IKON BEARING DISTRICT. 



ellipsoids. Others, iiml by far the greater number, have one set of lines par- 

 allel and another })arallel set which in different cases cut the first set at dif- 

 ferent angles, very rarely at a right angle (figs. 8, 9). One of these sets is 

 usually tranverse to the long axes of the ellipsoids (figs. 7, 8). 



The blocks are separated from one another b}' a thin layer of a schistose 

 matrix, rarelv moi'e than 3 inches in thickness, though exceptionally nearly 

 8 inches thick. (Cf figs. 7 and 8 of this paper, and fig. 1 by Ransome.-') 



Since the above description of the Crystal Falls ellipsoidal lavas was 

 written in 1896, there has appeared Sir Archibald Greikie's valuable work 

 on the Ancient Volcanoes of Great Britain,^ in which several similar occur- 

 rences are mentioned. His illustration of this structure on page 184, as can 

 readily be seen on comparison, would answer, but for the absence of a well- 

 defined schistose matrix between the ellipsoids, very well for 

 a sketch of a Michigan pre-Cambrian ellipsoidal lava. 



The schistose mati'ix between the elli23Soids upon the 

 weathered surface is seen to be made up of layers concentric 

 with the ellipsoids. It is possible that these layers are not 

 riG. g.-EUipsoiiis absolutely concentric in the third dimension. However, no 



'(vitb sets of parallel • i r i n • • n ^ * • i^ 



lines cutting each cxposurc permitted 01 the determination ot this point, t re- 



otheratanauglc. , . , J ir • j. j.1 c 



quentl}' certain layers seem to grade on into others ot a some- 

 what different character. The matrix between any two ellipsoids usually 

 separates near the center; where apparently the greatest movement having 

 occurred the schistosity is most developed. One can often as easily knock 

 an ellipsoid out of its encircling matrix as one can the kernel out of a nut. 

 Ill some cases there is no absolutely sharp line of demarcation between 

 matrix and ellii^soid, but a gradation from one into the other. At the places 

 where three blocks are in juxtaposition one frequently finds, instead of a 

 triangular space entirely filled by the matrix, in the center of the matrix a 

 triangular area of infiltrated vein quartz (figs. 7, 8). 



In certain cases the minerals which compose the schistose matrix are 

 not thoroughly cemented and gi^-e it a somewhat friable character, causing 

 it on weathered surface to appear granular. 



In very rare cases a matrix with a distinctly brecciated character was 

 observed, but in this as well as in the cases above described a certain degree 



I Op. cit., p. 76. 



" Ancieut Voleauoes of Great Brltaiu, by Sir Archibald Geikie, Vol. I, 1897, pp. 26, 184, 193. 



