220 The American Geologist. April, 189* 
The relative ages of the strata, inclosing the Mother Lode, can 
be ascertained with some accuracy. Of the five most common 
rock species : slate, diabase, serpentine, granite and syenite, the 
slate is the oldest, and all the others have been intruded through 
it after its elevation. It has been pushed aside and more or less 
broken b} T each of the others, and frequently portions are inclosed 
in the crystalline masses. 
Of the two, serpentine and diabase, the former though repre- 
senting the extreme stage of decomposition, is the younger. The 
proof of this may be found in the intrusion of a long, narrow dike 
of serpentine through the diabase of mount Bullion. The age of 
all the diabase dikes is proximately the same, judging from their 
lithological similarity, position, and the amount of dynamical 
metamorphism undergone. 
It is difficult to account for the great amount of .conglomerate 
which almost always forms a part of the diabase. It has been 
shown conclusively that it is not of sedimentary origin, by the 
presence of anrygdules in the matrix as well as in the pebbles. It 
has not the character of a friction-breccia for it does not alwa}'S 
appear near the edge of the dike, and the greater portion of it con- 
sists of rounded pebbles, the fragments of petrosilex alone being 
angular. Neither is it possible that the fragmental character is 
the result of a surface outburst, for the portions exposed at present 
were perhaps thousands of feet below the surface at the time of 
the intrusion. It must then have resulted from the breaking up, 
at a great distance below the surface of a bod} T of previously ex- 
isting diabase, the fragments of which were moved about in the 
molten mass till rounded. The presence of diabase pebbles in the 
sedimentary rocks can only be accounted for by the supposition of 
the existence of a body of diabase previous to the present one. 
The age of the granite is certainly less than that of the slates 
and probably less than that of the serpentine ; not only are the 
contact phenomena well illustrated in the outburst of the granite 
through the slates but the serpentine is bent out of its normal 
position so that it forms nearl}" a semicircle. 
The large areas of gneiss lying east of the lode are undoubtedly 
of metamorphic origin. In them the bedding planes represent 
those of the original sediments. The gradual change of these 
gneisses into an uncrystalline schist, and their correspondence in 
dip and strike to the schists are also strong evidences of such an 
origin. 
