228 SIR ARCHIBALD GEIKIE ON THE BASIC AND [May 1 894, 



the sake of the progress of geology I sincerely regret. Though 

 doubtless much has still to be learnt regarding the behaviour of 

 eruptive masses of igneous rocks, there are certain phenomena of 

 which so many examples have been observed that they are generally 

 considered to be well established. Among these phenomena are 

 the felsitic, spherulitic, and flow-structures which are found in 

 lavas and in the peripheral and apophysal portions of deep-seated 

 bosses, at the margins of sills, and often in the body of dykes and 

 veins. These structures have been so abundantly noticed under cir- 

 cumstances pointing to the effects of cooling and consolidation, 

 that, though some of the details of the processes still need elucida- 

 tion, the general conclusion has been accepted that the structures 

 represent stages in the comparatively rapid chilling and solidifica- 

 tion of molten material. The student who has mastered this part 

 of tectonic geology, and who has become familiar with examples of 

 these effects of cooling among rocks in the field, may well feel per- 

 plexed when Prof. Judd tells him that these very structures can be 

 produced by re-fusion and such excessively slow cooling as must 

 take place within a deep-seated basic eruption ; and that blocks of 

 granite, several yards in diameter, may in such a position be melted, 

 and instead of assuming a distinctly crystalline structure, as it 

 might have been supposed that, under these conditions of prolonged 

 high temperature and extremely gradual refrigeration, they would 

 have done, may acquire a fine felsitic texture, an exquisitely perfect 

 spherulitic flow-structure, conforming to the surface of the enclosing 

 material and presenting all the usual sigus of true rhyolitic move- 

 ment. His bewilderment will be still further increased when he 

 learns from Prof. Jadd that this alleged order of change is entirely 

 borne out. by microscopical investigation. He may surely be par- 

 doned if he feels inclined to abandon in despair a subject wherein 

 the testimony of field-evidence and of microscopic research may be 

 made so entirely to contradict common experience. 



Happily the conflict of testimony is not to be' found in nature. 

 I have shown, I think, conclusively that the whole of the phenomena 

 in Skye are perfectly harmonious and intelligible, that they involve 

 no exceptional occurrences, but that they exhibit in a singularly 

 clear and striking manner the behaviour of an acid rock which has 

 disrupted and invaded an older basic group. The literature of 

 inclusions in igneous rocks cited by Prof. Judd, and his detailed 

 account of the quartz-felsite fragments of Ascherhiibel, though 

 interesting in themselves, are really irrelevant to the facts observable 

 at Druim an Eidhne and its neighbourhood. In the field he has 

 missed the clear evidence of the dykes proceeding from the grano- 

 phyre, and. following his original error, has reversed the true order 

 of sequence of the volcanic rocks. With the microscope, the in- 

 fluence of the same unfortunate misreading has led him to invert 

 the actual succession of structures. 



The locality in Skye which he has now brought forward to prove 

 his contention is only one of many which I have cited in support 

 of the view that the acid protrusions are among the latest of the 



