Other formats

    TEI XML file   ePub eBook file  

Connect

    mail icontwitter iconBlogspot iconrss icon

Geology of the Provinces of Canterbury and Westland, New Zealand : a report comprising the results of official explorations

Igneous Rocks

Igneous Rocks.

I have already pointed out that during the deposition of the upper calcareous strata, eruptions of basic rocks have taken place. At the Limestone Bluff, amongst other localities, beds of lapilli and ashes have been formed between these eruptions, on the southern side of the Ashburton. In other localities, these volcanic tufas are represented by palagonite tufa, of which a specimen was analysed by the Chevalier Charles de Hauer, the results showing that it corresponds very well with the analysis of specimens obtained in other countries. *

I think that the dykes found at the entrance of the north Ashburton, consisting of a basic (doleritic) rock, at the gorge of the Rakaia, and at the Acheron, where the rock has a more granitoid texture, have been erupted about this time. More to the south, the submarine eruptions have taken place on a still larger scale, of which the dolerite plateau page 314upon the lower end of which the town of Timaru is situated, is the largest remnant. The best section to be obtained of the relations between the dolerites and calcareous rocks, also forming here the uppermost beds of the Oamaru formation, is in Mount Horrible, about 12 miles from Timaru, rising 1138 feet above the sea-level. From here to the sea, the coulées of dolerite can be traced almost without intermission, showing first that they gradually get thinner as we advance towards the coast, and also that their fall is very slight, being for the last seven miles only about 90 feet in the mile, or 1 foot in 58, the inclination indeed being almost imperceptible. Thus one of the streams, which on Mount Horrible, where they appear almost horizontal, is about 50 feet thick dwindles down to four feet near Timaru. The streams consist of a basic rock (Anamesite), porous, and resembling the uppermost beds in the Malvern Hills. Upon tbe calcareous greensand, forming the upper bed of the Oamaru formation, and which is often so rich in carbonate of lime that it can be used for the limekiln, a bed of volcanic tufa reposes, sometimes changing into an agglomerate, after which the first lava-stream appears. Then follow a number of tufaceous beds with some smaller lava streams between them, till the uppermost coulée is reached, having like the lowest one also a thickness of about 50 feet. This anamesitic rock is extensively quarried near Timaru, and forms a valuable building stone for that town. A similar coulée occurs at the Waihi Bush, between the River Waihi and Hae-Hae te Moana creek, both affluents of the Opihi. There also it covers strata of a calcareous nature, belonging to the Oamaru formation. More to the south, no volcanic rocks of any kind are to be observed, but in the greensands underlying the calcareous beds, lapilli of basic lava are not unfrequent. They might have some connection with the volcanic eruptions during which the lava-streams near the Otepopo river in Otago were ejected, and which are found in a similar position. There is thus sufficient evidence that submarine volcanic eruptions were of not unfrequent occurrence in our tertiary seas.