210 
LACCOLITHIC GENESIS 
and in the coal-fields of Germany and Hungary Moeste^® and 
Vom Rath describe like occurrences. 
The distinctive features of the sill as compared with those of 
the laccolith are shown in the following diagram (figure 14). 
Mechanism oe Laccolithic Location 
Concerning the cause of laccolithic intrusion Gilbert did not 
lose sight of certain mechanical shortcomings of his explanation. 
These he sought to overcome by appealing to factors which later 
Cross showed to be both unnecessary and not demonstrated as 
such. Dana got over the difficulties by brushing aside all con¬ 
siderations except simple hydrostatic pressure and with this fea¬ 
ture alone regarded Gilbert’s hypothesis complete. This is doubt¬ 
less one of the main reasons why from a mechanical angle lead¬ 
ing European geologists have so persistently challenged the Amer¬ 
ican view of laccolithic intrusion. At the same time Old World 
writers on the theme offer no alternative theory to take the place 
of the one which they seek to discredit. In the Sierra del Oro 
the chief objections which were raised against the Gilbert view 
seem to be fully met. Controlling tectonic features which all 
describers of laccoliths have missed thus appear to supply the 
long sought desiderata. 
In order that a laccolith be produced rather than any other 
form of volcanic manifestation it appears that the intrusive mass 
shall have a particular tectonic setting. Profound faulting is one 
of the prime factors. Another is orographic flexing by which the 
rigidity of certain arching strata largely maintains the load of 
superincumbent materials. Probably the high viscosity of acidic 
18 Geologische Schiederung Meisneru. Hirschberge, 1867. 
19 Neues Jahrbuch, f. 1880, p. 276. 
