REPORT OF THE CHIEF ASTRONOMER 747 



SESSIONAL PAPER No. 25a 



conclude that a xenolith, even very slightly denser than such a plutonic magma, 

 will sink into it. Since such magmas necessarily cool with extreme slowness, 

 there is evidently good ground for believing that an enormous amount of solid 

 rock could be engulfed before practical rigidity is established. The average 

 xenolith must sink in a less dense magma with the viscosity of pitch — yet how 

 much more rapidly in magma possessing the low viscosity which is postulated 

 in any of the ruling theories of plutonic-rock genesis ! 



Rise of Magma through Stoping. — We may legitimately imagine that a 

 shell of country-rock, say 100 feet thick, is thus stoped out of the roof of a 

 batholithic chamber. The rock at the new molar contact must undergo similar 

 one-sided heating and the stoping process is continued. By the summation 

 of these relatively small effects the upper level of the magma would be raised, 

 so long as the original supply of heat held out, unless the roof were finally 

 punctured and engulfed. If the heat supply did not suffice to produce such a 

 catastrophe, the form of the batholithic chamber would be that of a downwardly 

 enlarging compartment within the invaded formation, though a pipe-like 

 chamber could also be produced. 



Stoping will vary in rapidity with the size of the blocks rifted. The 

 average block near visible contacts is most probably smaller than the average 

 block rifted during the much longer period of high fluidity in the magma. 



But the development of the magmatic chamber is, after all, not so important 

 for petrogenie theory as is the fate of the engulfed blocks. Nearly all of these 

 must certainly be dissolved as long as the great mass of magma remains 

 fluid. Such abyssal assimilation means the wholesale formation of new, secon- 

 dary magma. This topic will be treated in a following section. 



Testimony of Laccoliths. — In view of the extreme improbability 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 stoping. Of 

 course, the conditions for rifting and for the submergence of blocks from the 

 roof, are much less favourable in the rapidly intruded magma of a typical 

 laccolith than they would be in a deeper-seated magma in direct communication 

 with the ' ewige Teufe.' Some notable degree of viscosity seems necessarily 

 assumed as characteristic of laccolith magmas. The proved laccoliths are all 

 small and are surrounded 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 

 unlikely that total digestion would destroy blocks fallen from the roof. They 

 might, therefore, be looked for on the floors. So far, the writer has discovered 

 no evidence on the point in any of the monographs. The reasons are not far 

 to seek. Very few floors of laccoliths are actually exposed. It is probable, too, 

 that in many instances an observer would have difficulty in distinguishing blocks 



