DIFFERENTIATION IN MAGMA BASINS 555 



rock types. Bo wen remarks that the upper acid magma, remaining 

 fluid after the lower basic portion has wholly crystallized, may come 

 to have an intrusive relation to the latter. This would be sufficient 

 to explain veining of the one rock by the other; but, where an 

 overlying sheet is separated from an underlying one by a surface 

 of discontinuity, I can see no explanation but that of distinct intru- 

 sions. Nor is this explanation necessarily excluded even when 

 no sharp division is seen, for, under appropriate conditions, a 

 transitional zone may result from partial admixture. The Sudbury 

 laccolite is probably a case in point, though I must confess to only a 

 limited personal examination of the mass. I found no indication of 

 a regular "composition gradient" in either norite or granophyre, 

 considered separately, while the transitional zone between them 

 has all the characters of a hybrid rock. The sulphide ore I leave 

 out of count, as doubtless representing a magma immiscible with 

 that of the norite. The clear instances of gravitative differentiation 

 in sills and laccolites of which I have direct knowledge are all in 

 rocks which must represent very unusually fluid magmas, such as 

 the analcime-bearing intrusions of Permian age in Scotland. They 

 are the kind of exceptions which help to prove the rule: viz., that 

 in an intrusive body of moderate size a prohibitive viscosity soon 

 puts a stop to the settling down of crystals. Doubtless a laccolitic 

 mass of very large dimensions retains its fluidity longer, but it is 

 obviously in a great intercrustal reservoir that the most favorable 

 conditions for this action will be realized. 



Bo wen would apply the conception of differentiation in place 

 to the plutonic rocks of Skye; but the facts, as I see them, abso- 

 lutely negative such a hypothesis. The peridotite is not found at 

 the base of the gabbro, but enveloped in the midst of it. The 

 granite breaks through the gabbro, and, where any approach to a 

 stratiform arrangement is apparent, does not overlie, but underlies, 

 the basic rock. In one part the granite has been so chilled against 

 the gabbro that its margin and the offshoots from it assume the 

 characters of a spherulitic rhyolite. I infer that the gabbro was, 

 in this place, not only solid but cold when the granite was intruded. 

 The large gabbro laccolite itself is made up of numerous irregular 

 sheets, showing differences in composition and structure, and often 



