FERGUSON AND TURGEON: HARNEY GRANITE. 281 
For this reason all the large sills of the area were found in the vieinity 
of Sandy Creek, while only small thin sills occur on the top of the dome. 
Of interest in this connection are the experiments made by Howe, 
(Jaggar, p. 291-303) where laccoliths were formed of molten wax 
injected into a sedimentary series built up of thin layers of plaster, 
coal dust, marble dust, and sand. Here it was found that where the 
injection was rapid there was a marked tendency to form sills. (See 
especially experiments 3, p. 297, and 5, p. 300, and Plate 18, figures 
3 and 5.) 
Summary.— The outcrop of Algonkian above the porphyry on the 
side of the gulch is of interest on account of the presence of a schist 
different from those found in the vicinity and of granite identical with 
that found in the Harney Peak region many miles south. ‘To account 
for this, two explanations — neither entirely satisfactory — have been 
suggested. It may be a part of the floor of the laccolith, carried up by 
the porphyry with the Cambrian in place above it; but if so it is difh- 
cult to account satisfactorily for the difference between this schist 
and that of Deadwood, only a few miles away. On the other hand 
it may be an inclusion from great depths, representing a part of the 
Algonkian lower than is exposed at Deadwood, brought up by the 
porphyry and “frozen” against the Cambrian. If this is the case it 
seems strange that there should be no porphyry between the Algonkian 
and the overlying Cambrian. 
The presence of porphyry in sills in the Minnelusa formation is 
noteworthy as showing that, in this laccolith, the intruding magma 
reached a higher horizon than had been supposed. The fact of the 
sills being of greater thickness and extent in the syncline than in the 
anticline is explainable by the supposition that the laccolithie dome 
had already been formed before the intrusion of this porphyry and 
hence the syncline already formed was a favorable situation for later 
intrusion. 
