File:The Quarterly journal of the Geological Society of London (1914) (14757466286).jpg

Identifier: quarterlyjourna701914geol (find matches)
Title: The Quarterly journal of the Geological Society of London
Year: 1845 (1840s)
Authors: Geological Society of London
Subjects: Geology
Publisher: London (etc.)
Contributing Library: Smithsonian Institution Libraries
Digitizing Sponsor: Biodiversity Heritage Library

View Book Page: Book Viewer
About This Book: Catalog Entry
View All Images: All Images From Book
Click here to view book online to see this illustration in context in a browseable online version of this book.

Text Appearing Before Image:
nclude a trachyte, three trachytoid phonolitos,one kaiwekite, two trachydolerites, and numerous basalts. It is,therefore, evident that a complete examination of the differentlavas might be expected to throw a considerable and perhapsimportant light upon the causes which promote those differencesthat are sometimes found in the succession of lavas that issue froma single volcano, or at least in the same volcanic district. In the first place, evidence was sought with the object of ascer-taining whether the various lavas which arc exposed in the cliff Vol. 70.) LAYAS AT THE NORTH HEAD, OTAGO HARBOUR. 383 were the products of the activity of a single volcano. Some diffi-culty was experienced in coining to a conclusion on this point,since all the outward signs of volcanic action have now beendestroyed by subaerial erosion, and therefore the surface-featuresthat exist do not indicate the point from which the lavas wereemitted. Internal evidence also was sought in the dip of the various flows
Text Appearing After Image:
of lava and in the extent to which they suffered from erosionduring the intervals between the different periods of activity. Thedip or slope of the lavas was found to be remarkably uniform.The amount of dip is 14-6°, and is directed 20° south of east. Therise is thus 1 in 4, a little to the north of west. This rise is inthe direction of the high country (1600 feet) near Mihiwaka andMopanui; but it is considerably steeper than the actual averagerise of the countrv in the same direction. 2d2 384 THE SEQUENCE OF LAVAS AT THE XORTH HEAD. (Dec. I914. In general, the thickness of each lava-flow is fairly constant. Itis certainly constant enough to warrant the conclusion that all thedifferent streams of lava flowed down the same slope, and pre-sumably, therefore, they were emitted from the same orifice. Inevery case a scoria-bed of some thickness covers the lava. Thematerial of the scoria is moderately coarse, as a rule, although thatwhich overlies the lowest phonolite is much finer than t

Note About Images