474 Reports and Proceedings — 



The intruded rocks also throw much light on past earth-history, and 

 I cannot give a better illustration of the valuable information which fchey 

 may furnish to the stratigraphical geologist, when rightly studied, than 

 by referring to the excellent and suggestive work by my colleague, 

 Mr. Alfred Harker, on the Bala Volcanic Rocks of Carnarvonshire. 1 



Perhaps the most striking instance of the effect which detailed strati- 

 graphical work has produced on geological thought is supplied by the study 

 of the crystalline schists. Our knowledge of the groat bulk of the rocks 

 which enter into the formation of a schistose complex is not very great, 

 but the mode of production of many of them is now well known, and the 

 crude speculations of some of the early geologists are now making way for 

 theories founded on careful and minute observations in the held as well as 

 in the laboratory. Eecent work amongst the crystalline schists shows, 

 furthermore, how careful we should be not to assume that because we have 

 got at the truth we have therefore ascertained the whole truth. We all 

 remember how potent a factor dynamic metamorphism was supposed to be. 

 owing to discoveries made in the greatly disturbed rocks of Scotland and 

 Switzerland ; and the action of heat was almost ignored by some writers, 

 except as a minor factor, in the production of metamorphic change. The 

 latest studies amongst the foliated rocks tend to show that heat does play 

 a most important part in the manufacture of schists. The detailed work 

 of Mr. George Barrow, in North-east Forfarshire, 2 has already thrown 

 a flood of light upon the origin of certain schists, and their connection 

 with igueous rocks, and geologists will look forward with eagerness to 

 further studies of the puzzling Highland rocks by this keen observer. 



The subject of former climatic conditions is one in which the geologist 

 has very largely depended upon followers of other branches of science for 

 light, and yet it is one peculiarly within the domain of the stratigraphical 

 geologist ; and information which has already been furnished concerning 

 former climatic conditions, as the result of careful study of the strata, is 

 probably only an earnest of what is to follow when the specialist in 

 climatology pays attention to the records of the rocks, and avoids the 

 theories elaborated in the student's sanctum. .The recognition of an Ice 

 Age in Pleistocene times at once proved the fallacy of the supposition that 

 there has been a gradual fall in temperature throughout geological ages 

 without any subsequent rise, and accordingly most theories which have 

 been put forward to account for former climatic change have been advanced 

 with special reference to the Glacial period or periods, although there are 

 many other interesting matters connected with climate with which the 

 geologist has to deal. Nevertheless, the occurrence of Glacial periods is a 

 matter of very great interest, and one which has deservedly received much 

 attention, though the extremely plausible hypothesis of Croll, and the 

 clear manner in which it has been presented to general readers, tended to 

 throw other views into the shade, until quite recently, when this hypothesis 

 has been controverted from the point of view of the physicist. In the 

 meantime considerable advance has been made in our actual knowledge, 

 and this year, probably for the first time, and as the result of the masterly 

 resume of Professor Edgworth David, 3 the bulk of British geologists is 

 prepared to admit that there has been more than one Glacial period, and 

 that the evidence of glacial conditions in the southern hemisphere in 

 Permo-Carboniferous times is established. Croll's hypothesis of course 

 requires the recurrence of Glacial periods, but leaving out of account 

 arguments not of a geological character, which have been advanced against 



1 Alfred Harker, Sedgwick Essay for 1888 (Camb. Univ. Press, 1889). 



2 G. Barrow, Quart. Journ. Geol. Soc, vol. xlix (1893), p. 330. 



3 T. "W. E. David, " Evidences of Glacial Action in Australia in Permo- 

 Carboniferous Time": Quart. Journ. Geol. Soc, vol. hi, p. 289. 



