﻿9§ NEW YORK STATE MUSEUM 



in diameter. They have a core of reddish orthoclase and a rim 

 of green oligoclase and are very characteristic. The granite 

 weathers badly and for this reason has received the Finnish name 

 rapakivi or rotten stone. Wherever these porphyritic granites 

 or even finer-grained porphyritic rocks are found with the zonal 

 phenocrysts, they are called throughout Scandinavia rapakivi. 



Now, the Finnish rapakivi is later than the post-Jatulian folding. 

 The rapakivi can be traced across the Aland group of islands which 

 separate the Baltic sea from the Gulf of Bothnia and themselves 

 have rapakivi rocks. It finds parallels or representatives farther 

 north along the east coast of middle Sweden near Sundsvall, and 

 inland about 60 miles at Ragunda. In this latter region the granitic 

 rocks of coarse texture lie beneath the Jotnian ; but their texture 

 shows that they crystallized beneath a load which was eroded before 

 the Jotnian sandstone was deposited. Thus we have a long time 

 interval between the close of the Jatulian period and the beginning 

 of Jotnian sedimentation. If now we establish the geological date 

 of the Swedish rapakivi, near Sundsvall, we also fix the time of 

 various other types of igneous rocks, such as diabases, gabbros and 

 granites, which cut the rapakivi but precede the Jotnian sandstone. 

 Extremely interesting exposures of all these were shown to the 

 visiting geologists at and near Ragunda, Sundsvall and north along 

 the coast in Nordingra, and are described in the guidebooks to 

 Excursion A 2. The visitors viewed with growing enthusiasm the 

 impressive phenomena of interrelated igneous types which Professor 

 Hogbom spread before us, nor will we ever forget the deep impres- 

 sion made by them. 



The most extensive of the Jotnian exposures, as stated above, lies 

 in western middle Sweden along the Norwegian boundary. The 

 underlying rock consists largely of the famous Dala-porphyries, 

 which are a subject of much difference of opinion. Some think 

 them Archean ; others are not convinced that this is true and urge 

 a later age. 



Near Sundsvall, which is a city in eastern central Sweden, and 

 the most important lumber center of the country, there is the island 

 of Alno, with its famous nephelite rocks, whose age is uncertain. 

 In many places throughout Sweden there are diabase dikes whose 

 age is also indefinite. It is conceivable that they may belong in 

 this interval between the Jotnian and Jatulian. 



New York can not furnish parallels for all the formations just 

 reviewed. As earlier stated, the Jotnian seems nearest akin to the 



