I40 The American Geologist. September, 1904. 
anorthite, but the average rock of the area is a normal gabbro 
composed of moiioclinic pyroxene, brown hornblende and bas- 
ic plagioclase. In some specimens there are large porphyritic 
hornblendes more than an inch long." Except for the mono- 
clinic pyroxenes, the noryte of Willow Creek is very similat 
to that of Dehcsa. A partial analysis of the former shows a 
close resemblance to an analysis of the latter. 
Dr. Chrustschoff, who has made a study of many forms 
of nodules in crystalline rocks, has divided them into four 
classes : 
1. Concentric, spheroidal and concretionary growths about 
foreign inclusions. 
2. Nodular growths about fragments of secretions or in- 
clusions which latter are often partially or wholly redissolved. 
3. Groups of so-called pudding granites where the struc- 
ture is due to a simple concretionary action, set up in the mag- 
ma during its normal crystallization. 
4. Primary structural fomis of the magma or endomor- 
phic contact products.* 
The explanation of Vogelsang,! in his discussion of the 
dioryte of Corsica, is as follows : 
"When a molten magma consolidates, an irregular (ungeleichmas- 
sig) cooling may produce greater contraction of the mass at certain 
points ; and this may lead later on to a spheroidal separation. If this 
condition is arrived at after the point of consolidation of the several 
minerals has been passed, and, tlierefore after their separation is com- 
plete, we get, indeed, a concentrically laminated body, but one without 
definite arrangement of the constituents ; this is the well known spher- 
oidal structure of many eruptive rocks. If on the other hand, the 
tendency to form spheroids is developed during the period in which a 
differentiation of the magma into its various minerals can still take 
place, the latter v.-ill undergo a definite arraingement with regard to 
the central point." 
It would seem that we have the two cases represented in the 
forms of orbicular gabbro from San Diego. The fact, how- 
ever, that the boulders are not in place, makes it impossible 
to conjecture on the position of the different varieties with lef- 
erence to the cooling surface. It is evident that the condition-; 
were not the same when the different varieties w^ere formed. 
* Memoires del. Acadeniie Imperiale des Sciences de St. Petersbourg, VII 
Series, tome xlii. No. 3. (Quoted from F. D. Ad.\ms' paper, the oriyinial by Dr. 
Chrutschoff is not accessible to the writers.) 
t Loc. cit. 
