AUTHOE'S PEEFACE. XIX 



the old centers, however, is by no means impossible. In a considerable 

 number of cases the larger and more important centers are still discernible, 

 though some are doubtful and exceedingly indistinct. The obscurity prob- 

 ably arises in many cases from the fact that while the greater accumula- 

 tions of lavas outflowed from great central vents or from loci within which 

 numerous vents were thickly clustered in close proximity, there were 

 numberless scattered orifices from which a few eruptions or even a single 

 eruption took place. And these dispersed vents were probably scattered 

 about in the intervals between the central localities of eruption. Such 

 craters would in the lapse of ages be wholly obliterated, and their out- 

 poured masses reduced to mere remnants The general effect of secular 

 decay has been to level the volcanic piles and build up the lowlands with 

 the debris. On the other hand, the great faults have brought up to daylight 

 masses of bedded lavas which otherwise would have been concealed, and 

 erosion has in many places attacked the faulted edges of the upraised 

 blocks and sawed deep ravines and chasms in which the igneous masses are 

 tolerably well displayed. Thus we are enabled to gain information con- 

 cerning the location of the centers of eruption which would otherwise have 

 been unattainable. But the knowledge so gained is far less perfect than is 

 desirable. 



Although it may seem that an investigation of such importance ought 

 to be easy, it is by no means so. The vastness of the masses displayed at 

 any center of eruption is such that no conception of their totality or of their 

 general arrangement can be gained without a somewhat protracted investi- 

 gation of a large area. But so rugged and formidable are the physical 

 features that such an investigation is about as difficult an undertaking as ever 

 falls to the lot of a geologist. 



The petrographic work has not been embodied in this volume. It has 

 not yet been completed, though considerable progress has been made. 

 Yet if it had been practicable to obtain the means to prosecute this branch 

 of research to the end, and to publish the results in such form and with 

 such illustration as the scientific student of the present day demands, it 

 would have been done. It was originally intended to make a thorough 

 series of chemical analyses of the volcanic rocks of this district. Many 



