526 abstracts: geology 



from which they are obtained, all of the conclusions to be ascertained 

 by means of the method of oblique illumination can also be derived 

 with even greater facility from the interference figure. For the study 

 of interference phenomena the method of oblique illumination does not 

 offer any special advantages over the convergent polarized light method, 

 but it does present certain disadvantages in manipulation and in the 

 distinctness of the phenomena observed which cannot be disregarded 

 entirely. This is especially true if oblique illumination be obtained by 

 use of a stop in the eye circle of the ocular as recently suggested by 

 Schneiderhohn. 



In this paper, the phenomena produced by oblique illumination are 

 discussed in some detail. Attention is directed to an obvious but im- 

 portant fact, too often disregarded in petrographic microscope work, 

 that for the accurate measurement of extinction angles central illumina- 

 tion by parallel plane-polarized light is highly essential. Satisfactory 

 measurements of extinction angles cannot be made when the section is 

 illuminated by a strongly convergent cone of light, incident under all 

 angles and in all azimuths, F. E. W. 



GEOLOGY. — The volcanic cycles in Sardinia. H. S. Washington. 

 Comptes Rendus Congres Geologique International XII, Toronto, 

 1913. 



The volcanoes of northwestern Sardinia were studied in the autumn 

 of 1905. They belong to three distinct periods, with interesting lavas, 

 which show very marked cycles, or recurrent successions of characters, 

 in their eruptions. These lavas are now being studied, and are to be 

 described soon, along with some forty analysed, in a series of papers. 



Taking these volcanoes as a text, some broad subjects of modern 

 petrology are discussed briefly. It is pointed out that, while no one 

 sequence of types is generally applicable, the sequence seems to vary 

 with the magmatic character and usually closes with basalts, tho any 

 generalizations must be rather hazardous owing to the inherently ac- 

 cidental character of the rock sequences observed by us. A change in 

 the character of the magma and in the volcanic cycles seems to be con- 

 nected with a change in the mode of volcanicity, the relation being pos- 

 sibly a causal one, but the inadequateness of present data for such studies 

 is pointed out. The so-called Atlantic and Pacific tribes of rocks are 

 briefly discussed and objections raised against them, it being urged 

 among other things that it is illogical and unjustified to select only two 

 types for contrast to the exclusion of others equally important. In con- 



