A Multiple Diabase Dyke. — Lawson. _'.'") 
sheets so as to anastomose ; and little apophyses of the dia- 
base in some cases branch out from the main sheet and die 
out in the granite. These facts, taken together with the pe- 
trographic identity of the diabase with that of the larger 
neighboring dykes, establishes the intrusive character of the 
sheets. A section across the edges of these alternating verti- 
cal sheets of granite and diabase was carefully measured. The 
section selected is a representative one, but neighboring sec- 
tions would be slightly different in detail, as the sheets are not 
uniformly thick along their strike and, as stated, the diabase 
sometimes cuts across the granite sheet obliquely. The re- 
sults of this measurement are as follows, in which the thick- 
ness is given in inches, d=diabase, g=granite: Dl^, g4. d4, 
g£, d2h gU. d-U, d3, g2, dl. gH, d6, g2, d3, g7, d'lh gU, d3, 
gl,d3, g3, do, g2, d3i, g-4, d2, g2i, d3i, glf, d3, g2, doj. g\. 
d4£, g2, dl, g2^, dl. g±, dl, g£, dl j, g2^, do, gU. d4, g4A. do. 
g8, d6, g3, d5, go, d2. 
We have thus in a breadth of about 14 feet no less than 28 
vertically intrusive sheets of diabase, ranging in thickness 
from 1 inch to fi-| inches, separated by 27 sheets of granite 
ranging in thickness from | inch to 8 inches. There can be 
no reasonable doubt that these diabase sheets represent 
but one invasion of magma and that they are essentially but 
one dyke, although split up into so many parallel sheets. 
Such dykes may conveniently be called multiple dykes. The 
extent of the dyke is not known. Its strike is about S. S. E. 
Its exposure is limited to the rock surface shown in the ac- 
companying photograph (Plate VII). The striping of the sur- 
face, due to the alternation of light and dark colored bands of 
rock, is fairly well brought out in the illustration. The band- 
ing is, however, even more uniform than appears in the pho- 
tograph, seeming irregularities being introduced by pools of 
water, clefts, shadows, etc. 
The interesting feature of the occurrence is the splitting 
of the granite into so many thin vertical sheets. The granite 
is seemingly homogeneous. There is no differentiation of 
structure or of mineral composition,— no trace of foliation. 
The splitting in parallel sheets would therefore seem to be 
due to no intrinsic structural feature of the rock itself, but 
to have been determined by the nature of the forces directed 
