Repokt of the State Geologist. 



they are the most northerly of the post-Utiea basic- dikes known along the 

 lake Champlain meridian. 



Dike No. 1 is exposed by the roadside on the road south from Platts- 

 burgh, a few yards beyond the point where it rises from the sand plain on to 

 the Bluff Point limestone exposures. It is a quite typical monchiquite, with 

 very abundant olivine phenocrysts, often perfectly fresh, which speck the 

 dense black rock with white spots. Occasional small porphyritic augites 

 also occur. The dike being so narrow, the ground-mass is very dense 

 throughout, and consists of small, slender augites, magnetite and sparse 

 brown hornblendes set in an apparent glassy base, most, if not all of which 

 has a very weak double refraction, and thorough investigation may show the 

 presence of analcite. 



Dike No. 4 cuts the Trenton north of Bluff Point, and is near the south 

 end of the section. It is largely made up of a nearly colorless augite, which 

 occurs in two generations, phenocrysts, however, being only occasional. 

 Olivine is present,, but rare, and can not be regarded as an essential 

 constituent. Hornblende is absent except for a few green, uralitic crystals 

 of secondary origin. Magnetite is present in considerable amount, and 

 there is also a notable quantity of biotite in the ground-mass, some of 

 which is certainly primary. The rock contains numerous round white spots, 

 giving it an amygdaloidal appearance. In the centre of the dike is a strip 

 six inches wide which is clearly marked off from the rest, runs the whole 

 length of the dike, and gives the impression of a second dike cutting the first. 

 The rock is of the same character, however, though the Avhite spots are less 

 abundant than in the main dike. Under the microscope these white 

 spheroids are seen to be formed of a colorless, isotropic mineral, which by 

 means of a gypsum plate, is seen to be optically anomalous, in that it shows 

 faint double refraction. The mineral seems to be analcite. Some of the 

 spheroids contain also calcite crystals. Inclusions of biotite, apatite and 

 hornblende, of rather large size, occur in the analcite. The spheroids are not 

 sharply bounded, but grade into the rock. Considerable analcite is also 

 present in the ground-mass of the rock."' 



Dike No. 19, from Crab island, is quite similar. It is made up of augite, 

 biotite and magnetite, with glassy base, but it lacks the analcite spheroids, and 

 instead is amygdaloidal, the cavities being tilled with calcite and zeolites 

 other than analcite. It has numerous and large porphyritic augites. 



* Since the above was transmitted for publication, the article published by Professor L. V. Pirssou " On the Monchiquite or 

 Analcite Group of Igneous ]<oeks*" has appeared (Journal of Cieology, Vol IV.. p. 679). Pirsson shows the presence of analcite 

 to be a fpatin-e of these rocks. The analcite In Dike No. 4 seems to be primary. 



