PETROLOGY IN AMERICA. 473 



cent, of silica and 1 1 per cent, of soda, and consists of 

 porphyritic crystals of nepheline up to an inch in diameter 

 in a tinguaitic ground-mass rich in nepheline and aegirine. 

 The associated basic dykes in this district are mostly rich in 

 biotite, corresponding with the ouachitite of Arkansas and 

 in some cases with the allied type fourchite. 



The dykes of the Lake Champlain district in New 

 York and Vermont were noticed by Kemp and Marsters 

 in 1 89 1, and more recently a Bulletin of the National 

 Geological Survey has given a fuller account by the 

 same writers (21). Besides bostonites and diabases, they 

 describe typical camptonites with hornblende, augite-camp- 

 tonites, monchiquites, and other rocks which from the 

 absence of olivine are placed under fourchite. Near Danby- 

 borough in Vermont, Marsters (22) has described a variety 

 of camptonite differing from the Campton type in having 

 no porphyritic hornblende. This mineral occurs in idio- 

 morphic brown crystals in the ground-mass, while there are 

 also two generations of augite and rather abundant biotite. 

 From Lake Memphremagog, on the Canadian border of 

 the same state, he has recorded (23) a number of lampro- 

 phyre dykes associated with dykes of granite, occasionally 

 taking on the characters of bostonite. The prevalent type 

 here is an augite-camptonite, in which both augite and horn- 

 blende occur in two generations. Hornblende -camptonite, 

 monchiquite, and fourchite are represented by single dykes. 



These various rocks, as will be noticed, do not contain 

 any mineral of the " felspathoid " group (leucite, nepheline, 

 sodalite, etc.). Kemp (24) has, however, drawn attention 

 to a peculiar dyke occurring at Hamburg, N.J., at some 

 distance from the Beemerville elaeolite-syenite, and present- 

 ing mineralogically some resemblance to the rather ill- 

 defined group of rocks named teschenites. The rock in 

 question consists nainly of biotite and pyroxene set in an 

 interstitial mass of analcime. It contains spheroidal bodies 

 up to 10 mm. in diameter composed of analcime. apparently 

 pseudomorphs after some vanished mineral. Hussak in 

 1892 had already taken these as indicating destroyed leucite, 

 and Kemp has subsequently placed this beyond doubt by 



