R. A. Daly — Mechanics of Igneous Intrusion. 285 



hand, that partial subsidence of graben-blocks at fault-troughs 

 can be proved. The former conclusion seems warranted for 

 the very numerous smaller stocks, and the writer believes that 

 it is in the main permissible, and even necessary, in the case of 

 most batholithic masses. 



Tests of the Hypothesis of Overhead Stoping. 



The hypothesis of magmatic overhead stoping is believed, 

 then, to be founded on a thoroughly competent geological 

 process, one for which there is a certain amount of clear ocular 

 demonstration in the field, namely, contact phenomena much 

 more widely disseminated in nature than the dislocation or 

 caustic phenomena yet proved on behalf, respectively, of the 

 " laccolithic " and "marginal assimilation" theories. It is 

 believed that neither of the latter theories lays enough empha- 

 sis on the normal property of internal plutonic contact belts of 

 systematically containing large numbers of fragments from the 

 invaded formations. The laccolithic theory suffers further 

 from the absence of observations proving the existence of base- 

 ments of country rocks for the larger granite massifs. The 

 hypothesis of stoping and rapid sinking explains the general 

 lack of such blocks in the central portions of the massifs, and 

 is corroborated by the facts, going to show that, precisely under 

 those conditions when blocks would not sink in the less dense 

 magma, the latter exhibits independent evidence of being 

 enormously weakened in its thermal, chemical, and physical 

 (rifting and stoping) activity. 



Testimony of laccoliths. — In view of the extreme improba- 

 bility that one can often, if ever, expect to find the pressure- 

 solid, or otherwise determined floor of a deep-seated magma 

 basin,* it is of interest to question the few known laccoliths 

 with visible floors for information as to the efficiency of stop- 

 ing. Of course, the conditions for rifting and for the snb- 

 mergence of blocks from the roof, are much less favorable in 

 the rapidly intruded magma of a typical laccolith from what 

 they would be in a deeper-seated magma in direct communica- 

 tion with the " ewige Teufe." Some notable degree of vis- 

 cosity seems necessarily assumed as characteristic of laccolith 

 magmas. The proved laccoliths are all small and are sur- 

 rounded on every side, except at the narrow conduit, by cold 

 rocks, so that chilling must be much more rapid than under 

 plutonic conditions. Nevertheless, the attempt has been made 

 to 'find, in the published descriptions of type laccoliths, any 

 statement for or against the probability of a limited amount of 

 rifting and stoping. In such small igneous bodies, it would be 



*Br6gger, op. cit., p. 134. 



