REPORT OF THE CHIEF ASTRONOMER 735 



SESSIONAL PAPER No. 25a 



whether any other sort of assimilation is possible than that by caustic or solvent 

 action of a magma on its roof and walls. Such information is found in the 

 same internal contact-belt where the general failure to prove solutional absorp- 

 tion of the country-rock has been so often reported. Within that belt it is the 

 rule to find often very numerous blocks of the invaded rocks. These have usually 

 "the following characteristics: varying size; angular or subangular outlines 

 against the eruptive rock, which is essentially unmodified even close to the con- 

 tact with each block; sharp contacts with the eruptive, in which the blocks are 

 completely immersed; a normally high crystallinity and increased density as a 

 result of contact metamorphism. Very often they show that they have moved 

 but short distances from the niches they once occupied in wall or roof. The 

 molar contact is similarly sharp. It may preserve, with exceeding definiteness, 

 the sharp corners left when the blocks were rifted off. Passing inwards, it is an 

 equally normal thing to find the foreign inclusions to become rapidly rarer, 

 until, in the heart of the eruptive area, one may go hundreds of yards or even 

 several miles without discovering any such inclusions. If there are hundreds of 

 them in a given part of the contact belt at the present surface (evidently a 

 chance section exposed by erosion), the natural inference that there are thousands 

 or millions of others enclosed in the eruptive below the level of the visible 

 contact, is clearly permissible. Another legion of them has been destroyed 

 along with their matrix in that part of the igneous body removed by denudation. 

 It is manifest, further, that the rifting of the blocks has so far enlarged the 

 chamber occupied by the eruptive. That is, the walls are, on the average, farther 

 apart because of the rifting. The question arises as to whether the chamber 

 may owe a large part of its present size to a long continuation of the self- 

 same process, with a simultaneous removal from the visible chamber of the 

 blocks formerly rifted off. The affirmative answer to this question is the kernel 

 of the hypothesis to be proposed. 



Strangely enough, the explanation of the presence of foreign blocks within 

 igneous bodies along the molar contacts and the equally conspicuous rarity of 

 such fragments toward the centres of the bodies, has only quite recently been 

 undertaken. How blocks still close to their former homes in the country rock 

 could be suspended in the magma until crystallization of the latter was complete, 

 and whether the normal effect of their complete immersion would be to permit 

 of their floating upwards or sinking downwards in the magma, are questions of 

 prime importance to the ensuing hypothesis. The attempt has been made to 

 answer them by correlating experimental and other data acquired for petrological 

 science within recent years. We may, for the present, assume the generally 

 accepted liquidity of normal plutonic magmas. 



Magmatic Shattering hy Differential Thermal Expansion. — A clear state- 

 ment of magmatic shattering has been given by Crosby in his monograph on 

 the Blue Hills Complex.* 



It is manifestly impossible to determine the exact rise of temperature which 

 will occur in a formation at the contact with an invading magma. Both ele- 



• Occasional Papers, Boston Soc. Nat. Hist., Vol. 4, 1900, p. 315. 



